There are a number of services that allow you to quickly build a social network without any hosting or software costs. There are:
What there services do is to allow you to register your community and create a basic site. with a unique URL. By directing your members to this URL you can immediately start building a profile.
There are back tools for administering the site.
In summary, its a quick and easy way of ruuning a social network however you loose control over advertising.
