I'd like to point out 2 things about why – IMO – Twitter won't be Facebook, never ever. And that's not a bad thing at all, don't get me wrong.
Twitter requires much more engagement to make sense and I don't think that the mainstream audience has the time / patience / interest in building meaningful conversations on Twitter. As a matter of a fact, Twitter serves pretty niche interests / segments and I'd be very happy to see it growing strong but still “confined” in those areas. More niches are what Twitter should really look forward to do, but aiming at the Facebook audience (ie ANYONE) simply doesn't make sense to me.

For sake of the argument, let's also say that the really mainstream websites aren't that much… google, bbc / nbc / etc, ebay, facebook, amazon, etc. But would you dare say that Flickr is mainstream? Hmmm, maybe (let's not only think about the tech world you belong if you read this blog / comment, let's think about real mass phenomenon). Is last.fm mainstream? Again, I'm not sure it is if we simply go out of the technorati sphere… So at the end of the day the notion of mainstream varies quite a lot.

Having said that, long life to twitter! And add me if you fancy, I'm http://twitter.com/fabiodebe/