One of these days Twitter is going to have to stop being a hobby and become a business.
When it does, presumably it will make its money from advertising. And when that happens, its numbers are going to be critically important to advertisers in determining value.
Right now, I have no idea how reliable Twitter's user numbers are. But I do know one thing first hand -- the number of followers ascribed to those users is completely unreliable.
When I first signed up for Twitter, and knew nothing about it, I checked a box somewhere that automatically had me follow everyone who followed me. I have no idea why I did that. I guess I thought it was Twitter etiquette.
Consequently, I am now following about 7,000 people. Except I am not. In fact, I only follow about 10 people. I don't know who those other 6,990 even are. I never look at their tweets, and I have tried my best to get rid of them, and can't.
You see, Twitter makes it as difficult as possible to stop following people once you start. In order to stop following 6,990 people you have to unfollow them one at a time. Which means I'd have to devote 10 years of my life to this. It's so much easier to just continue not-really-following them.
Not only that, when you try to find a third party program that has an easy way to stop following in bulk, you usually find that Twitter has intervened to stop the third party from distributing this program.
The only reason I can imagine Twitter is doing this is that they are trying to artificially maintain follower numbers so they can monetize them.
If Twitter's user numbers are as bogus as its follower numbers, it may continue to be a hobby for a long, long time.