Twitter was obviously a great idea, but it wasn't an obviously great idea.
Just because no one (including the Twitter guys) could see the direction it would grow in doesn't mean that it wasn't a great place to start. In retrospect, clearly it was.
I guess I don't think there is a 100% correlation between success and a great idea. I think that luck can be just as big a factor in success as the idea, especially in the mass consumer space.
If celebrities didn't take to Twitter I very much doubt they would have hundreds of millions of users now. You could argue the idea is what drove celebrities to use the service, but did it? If another service came out before or around the same time that celebrities used instead, would we say "Twitter is obviously a great idea that just didn't catch on?"
I think the thing that is being overlooked with Twitter and celebrities, is that it's yet another case of cutting out the middle-man that the internet does so well.
Delivery of celebrity gossip, information, photos to fans is big business. It was also gate-kept by an industry who decided who was cool and who was not.
Celebrities quickly worked out they could by-pass this channel and go direct to their fans. The broadcast and print media no longer were the gatekeepers, and the celebrities could build their own direct channel to their fans.
Now celebrities make money just by having a direct link to their fans and have an incredibly effective marketing channel. And now they often accept money for posting something on twitter.
I think it's one of those examples where the media-replacement is coming in along an axis that nobody expected.
The future of media is less in the masthead but more in the individually branded producer. To achieve this requires a lightweight direct channel. Which twitter has achieved without really setting out to.
Just because no one (including the Twitter guys) could see the direction it would grow in doesn't mean that it wasn't a great place to start. In retrospect, clearly it was.