Facebook is nearing it’s 1 billion user mark and it looks like twitter is not far from announcing it’s 500 millionth user. The issue will be though, how many of those user accounts are real and how many are fake. Fake accounts is a problem that twitter has faced for years and generally they refrain from announcing the number of users they have.
Instead they prefer to announce the number of active users on a monthly basis. Read more here: http://mashable.com/2012/07/30/twitter-users-500-million/