Earlier quoted context omitted.
Considering, I know nobody that actually uses Twitter I think 13% is wildly overstated.
Without wanting to sound too harsh: there's a reason why statistics are not based around personal anecdotes.
Officially, Twitter has 100 million active users – again, members who log in at least once a month. (January 13, 2012 6:00 AM) http://www.mediabistro.com/alltwitter/twitter-active-total-u...
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.