I think the problem with Twitter isn't necessarily even a problem with Twitter. It's a problem with all of the media types who're so quick to call it a game changing online revolution. The only thing that seems to be keeping Twitter intact is a hardcore group of early devotees, and then the rabble who have joined up recently in order to follow the celebrities who've taken a temporary interest in the service. Everything about it screams "trend," which is why I'm waiting to see which idiotic web powerhouse buys it for some inflated price, only to see it flop a few years later.
Tumblr, on the other hand, actually seems to have a lot more flexibility. You can build your own design from the ground up with CSS/HTML, a capability which most other blog services will charge you for, if they even offer it at all. It makes simple everything you ever wanted made simple by a blog service (i.e. posting pictures, videos, songs, etc). You can also send updates via SMS/email, if you prefer to run a Twitter-like thing. And you can create your own Tumblr RSS-style feed by "following" other users.
In the end, Twitter only has one thing, and that one thing is going to get old really quickly. Facebook and Tumblr have tons of functionality, and they're only adding more as time goes on. This isn't to say that Twitter couldn't add new functionality in the future, but they've already made their entire brand the 140-character "tweet." Not a very robust foundation from which to branch out, service-wise. And if they don't add new functionality, the gimmick is eventually going to wear off. I think the hardcore Twitter community would remain, but you obviously have a lot of people dipping their toes in these days, hoping that the thing will prove to be more than a gimmick, and actually branch out into other territories.