Michael Jackson's death crashed Twitter and several other online services, demonstrating the popularity of these things. But Twitter may be about to peak, as one company prepares to sell Twitter followers to advertisers.
Twitter is most interesting as an experiment in computing and social networking, with an emphasis on experiment. Twitter's popularity could diminish just as quickly as it rose if tweets start to be dominated by messages like "Buy Sugar Cola--It's good for you!"
Blogging has already passed its heyday. Blogging is not going away--in fact, it has proved to be an extraordinarily useful method of writing and disseminating news, information, and opinion. But hardly anyone still believes everyone will blog, and most now understand that blogs are just one more writing tool, and nothing more--a good writing tool, but that's it. Good blogs prosper because of good writers--just like every other kind of tool. Owning an expensive paintbrush does not make me Michaelangelo, and thankfully, we've passed through that phase of blogging where people thought a blog made them a good writer.
We're still trying to figure out what the long term purposes and uses of Twitter are--it's an interesting new tool, but not all of us need to tweet all day long.