That's just the obverse of leaving isn't it? The site loses any momentum, becomes stagnant, and peters out, or in the case of Friendfeed, became ripe for acqui-hiring, then effectively spun down.

Orkut actually kept growing for a while, but in secondary markets (not just Brazil, but that was its biggest geo), until started to stagnate then decline.

Digg is a great example as well.