Google Friend Connect API is supposed to help people to quickly set up dynamic social websites without digging too deep into Drupal etc. Kind of like that js-kit widgets but I would say a bit more comprehensive.
On the plus side is that you get the whole Open Social thing for free and whatever apps you may be interested in. The downside is that you have to send people off from the Google Friend Connect sites to Google for signup, and similar smaller things in the UI and user experience make it still somewhat clunky. For example, they have PhpBB, Wordpress and Drupal integration - perfect! - but having to prepend every FC username with "fc_" is already kind of weird, and not having 100% of control over user profiles, their look and feel, etc.... in other words, if you are looking for more flexibility, you will probably need a real CMS instead of this. Otherwise, Google Friend Connect API a pretty decent quick way to get a social site started.