It seems that “175 million users” may be a bit of an exaggeration. This article estimates the number of active Twitter users to be closer to 56 million. Although the math is a bit shady, it’s interesting to see the thought process that they went through.
Bonus fun fact:
At Facebook, a company source tells us, they believe that a user is not going to end up sticking around unless they make friends with 10 people.