I've seen a few migrations in my time. I wasn't a part of DIGG, but I did witness that happen. Reddit, if they're not careful, could go the same way for different reasons.

FARK messed up bad because they royally pissed off their user base over the course of a year or so. The fact that they were a smaller site and there was plenty of competition out there during that period didn't do them any favors.

Myspace kind of died on its own. Part of that was due to selling out, part of that was due to Facebook opening up their site to non-college users and offering a better service. Facebook just offered a better product and Myspace couldn't compete.

Facebook and Twitter are both kind of different. Facebook has a captive audience, because that's how a lot of people stay connected online. Quitting Facebook can have a literal, negative impact on people's social lives. Twitter on the other hand has a small active user base, but it's circled around people with strong social followings, like Celebrities and influential members of various sub groups. If they're to die down, it'll be through boredom and inactivity, or a better option eventually coming along, like what happened between Myspace and Facebook.