Just ran across this NY Times article on an emerging trend in social networks where Facebook and Twitter are openly and actively encouraging third parties to consume their services and re-purpose their content. The article then covers several startups who are feeding off of this social knowledge ecosystem.
With regards to this ecosystem, what I have seen in the wild are three patterns. A lot of UGC sites now permit you to post non-anonymously via some kind of federated login system. The second pattern is publishing content in multiple places such as micro-blogs on both facebook and twitter. The third pattern is profile synchronization across multiple social networking sites.
With federated login (sometimes called single sign on), you don't have to register at every site that requires registration. Instead, you can participate on a site as a registered user by logging in to an already existing account that you have. Under the covers the system, where you have an account, shares some knowledge about you with the system you are trying to use. Both Microsoft and Sun Microsystems tried their hand at federated login over a decade ago. Both failed miserably. Today's big players at single sign on are Google, Yahoo, and Facebook. Google and Yahoo use the OpenID technology to carry this off while Facebook uses its own Facebook Connect technology.
In the second pattern, you post some content and the system provides a way for you to immediately promote your content on the various popular social networking sites. Originally, this was called Viral Marketing where the system would volunteer to spam your friends if you would let it access the address book or contact lists from your main-stream email accounts. In this iteration, you are presented with options to send facebook notifications or tweets.
The need for the third pattern has risen only very recently as people start spending a lot of time updating their profiles on all of the social networking sites that they use. This trend is still fairly recent but two examples that come to mind are Disqus and Gravatar.
As to the startups referenced in the original NY Times article mentioned in paragraph one here, I would be a little nervous in investing in these companies. After all, they use the facebook and twitter back-ends as their main entre whereas these other companies see it more as the icing on the cake. I mean, what are they going to do if Facebook ever decides to shut down their free API?