As far as I'm aware, twitter has actually been a lot smaller in terms of users than you might imagine from its influence.
It has a relatively low number of active users, but the fact it's designed to be a centralised public forum (rather than users being selective who can follow them like Facebook) means it is/was very attractive for businesses, celebrities and politicians.
Also that -- thanks in large part to movements like the Arab Spring using Twitter to organize and publicize -- it became the go-to social media for reporters. The news -> celebrity -> news cycle closes itself nicely there, making it very difficult for either group to go anywhere else.