I don't think getting you to visit their site is the point of the switch. Twitter and FB are in a position where they have enough addicted users that they can get away with messing with standards. RSS is outdated and slow, and FB and Twitter want people to use their data in a more sensible way, via their flexible APIs.
It's trivial to build an APP that generates an RSS stream for an arbitrary twitter account with their API, and I'm sure someone will soon. But the point is that they are setting a new standard for what can be done streams of structured data, and hopefully more sites will move away from RSS into cool APIs.
Of course, for now at least, there's no standard to this kind of API, but I think that people will eventually figure out what works and converge to it (as is the case with OAuth).