Largest online social network
Who
Facebook
What
1,590,000,000 people
Where
Not Applicable
When

As of 31 December 2015, Facebook boasted 1.59 billion monthly active users, with 1.44 billion monthly mobile users and an average 1.04 billion daily active users.

Facebook was launched in 2004; it surpassed the 1 billion monthly active user milestone in 2012.