Exactly. The collaborative filtering technology & algorithms is the easy part. The hard part is getting mountains of good data to apply it to. And by good, I mean monetizeable.

That's what Amazon and Netflix have done. They should solve the data problem first, and being yet another "interactive social media mashup web 2.0 social networking thingy" may not be the best way to get lots of data. The companies that have done this best have monetizeable data - Amazon can actually SELL you the book it recommends.

As for being a collaborative filtering platform: "1. If your idea starts with 'We’re building a platform to ...' and you don’t have a billion dollars in capital, find a new idea. Now." (http://diffle-history.blogspot.com/2008/06/postmortem.html)