Facebook is already in decline, and will tank once something "better" comes along. The Twitter phenomenon isn't new... it's just the newest version of the same thing.
I think you might be confusing Facebook with MySpace since the former hasn't showed any signs of decline. In fact they've been growing at a fairly consistent rate for the past year or so and are nipping at Google's heels for that number 1 most visited spot on the net. MySpace on the other hand has been in decline for well over a year.
Would you care to cite a few examples of a Twitter-esque service that came before Twitter? The only other microblogging service that's been around almost as long as them (off the top of my head) is Tumblr and even they were launched about a year later. There were of course those engaged in microblogging before such services existed, but Twitter was still arguably the first to market.