WHAT IS YOUR PURPOSE?
There are many reasons for setting up a social network. These can include providing a space for members or supporters of your organization to interact with one another, sharing of expertise and experiences, building a community of knowledge, distributing information, allowing individuals and organizations to connect with peers, etc. Being clear about the purpose of your work is an important first step. One way to do this is by doing some brainstorming, talking with colleagues, and also reviewing other networks online to get some ideas.
In order to begin your own social network one of the most important decisions is what is the software platform/format that will be used and what type of functionality is provided, are there any costs, is the platform flexible, scaleable (can it grow as membership increases), etc. For this site I use a platform called NING (www.ning.com) which is wonderful company that provides platforms for social networks that have a high degree of customization. I have been doing this type of networking online for more than a decade and to date NING is my favorite resource. Ning used to provide their resources for free, recently they started charging monthly fees (they may still provide free sites to educators. I still think Ning is the best platform).
There are also many other platforms on which you can create a social networking site, some might require some programming expertise, while others are largely accessible to anyone who has a basic understanding of how a content management system functions (which is similar to using any of the main word processing software programs in which a web page can added/edits relatively easily). There are many open source contentment management systems that are available for free online. Some of the most reliable include www.drupal.org, www.joomla.org, www.mambo.org, www.plone.org. You can find a great deal online by searching and there are also aggregate sites that compare contentment management systems, such a CMS review or idealware/techsoup has a good review of some sites.. For an open source content management system, one of the steps that is required is finding a site or place to install, run and host the software (basically a server). This often requires some technical knowledge or there are some companies that will provide this service for a fee, which you can find by searching. Some people also choose to setup groups on Facebook or to setup parallel groups on Facebook.
In addition to NING and open source content management systems, there are some commercial products available for a fee. You can find these by searching.
Note if your goal is only to distribute information to your members, then signing up for one of the free listserv or group software platforms can be an effective and free way to do this. For example, YAHOOGROUPS or GOOGLE GROUPS allow anyone to setup a listserv and each provides some features where members can interact.
Also many organizations choose to build their own community sites by hiring expert IT companies or consultants (or hiring some to modify an open source system).
OPEN VERSUS CLOSED NETWORKS
An important question while creating a network is if the site should be open to anyone, moderated or a private network. This a vital decision that will affect who chooses to join the network.
Once you a clear goal and platform, one of the next steps is beginning to work on content. Even if you create the best site, unless you have strong content, the amount of traffic the site receives will be minimal. There is no magic recipe for creating content, but in general my recommendation is try to follow your passion in terms of creating content that fits with your own interests. But it equally important to try and ensure that you're not duplicating already existing content/resources. Try to see what is already available on the Internet and fill gaps, develop new areas, etc.
Building a member base is another key component of launching a network. Obviously one of the first places to recruit members is through your own colleagues and networks. Craft a short and clear e-mail inviting people to join the network (perhaps you might invite a few people to test the site before you launch the site publicly). Another valuable way is to post information about your site on other relevant sites/networks. For example, some key sites that might be appropriate for posting information include www.idealist.org, www.takingitglobal.org, www.developmentgateway.org, www.devzone.org. There are hundreds of other sites and lists where it is often possible to post free.
Another means of attracting members is to work out partnerships with like-minded sites/organizations and engage in cross-promotion of one another's networks.
Of course, starting a facebook group or a Twitter account can be other ways to attract members.
Some additional resources that maybe useful include:
TECHSOUP - TechSoup.org offers nonprofits a one-stop resource for technology needs by providing free information, resources, and support.
NPOWER - NPower is a network of locally based nonprofit organizations that provide comprehensive, high-quality and affordable technology assistance to other nonprofit groups nationally.
CASE Foundation Guide to Social Media for Nonprofits - Gear Up for Giving is series of social media tutorials, to help nonprofits and their supporters understand how to use key tools and techniques to create awareness, catalyze civic action and cultivate new supporters and donors for their causes. We've also scoured the web and compiled our favorite resources on some of the most popular social media tools, so you can learn more at your own pace.
CHARITYFOCUS - Since its inception in 1999, CharityFocus has partnered with hundreds of small nonprofit organizations (NPOs) to build custom web-solutions. CharityFocus' unique model enables volunteers to deploy a broad array of expertise to help NPOs better deliver services to their beneficiaries and more effectively reach their target audience. CharityFocus is completely volunteer run, and the services of its volunteers are absolutely free.
NETSQUARED - Today the global community stands witness to a momentous time in history where progressive change is not only necessary, but imminent. At NetSquared, we recognize that mandate and believe the social Web is key to making change. NetSquared works toward this goal by mobilizing individuals and communities, providing Web-based tools, and awarding financial support to leverage social action projects.
NETHOPE - NetHope is a nonprofit IT consortium of leading international NGOs serving tens of millions of endbeneficiaries each year in 150+ countries.
ICT for PEACEBUILDING - Exploring the use of information and communications technology for conflict transformation
ASHOKA's CHANGEMAKERS - Has published several useful guides for developing and promoting social networks including: A guide to social media, starting online groups, publicity and more.