Great example- the latest figures I can find state that only 13% of the US population use Twitter. I have to imagine the figure is even lower internationally, and only a tiny minority of that 13% post to it regularly.
However, I suspect a lot of those are bot's / sockpupets. Not necessarily spam, but a weather feed does not a twitter user make. I have seen estimates ranging from 24%-50% of all accounts are bots, and many users are over represented with a corporate account and a user account. Leaving perhaps 15-50 million actual users worldwide.
PS: I have been linked to tweets and I know plenty of people that do the same, but I don't have an account so by their accounting I am not a user and neither are the 100 or so other people I have talked to about it.
