Eventually, yes, I'd say so. GeoCities had a good 10+ year run. USENET lasted 20 years or so until it was overrun. Perhaps Facebook and Twitter can beat these, but they will certainly be killed off at some point, considering the rapid changes happening these days.
The key is that it's not a sudden shutdown -- it's the result of a painful decline over the span of a few years. The same thing will probably happen with FB and Twitter, as they lose users to the new thing.
