Well the next one below Facebook is Twitter at about half of Facebook users.
50% is significant decimal percentage.
Followed by Qzone at about 37% the size of Facebook which while focused on Chinese speaking people is still alot of people.
Can't count Instagram
Followed by Tumblr and VK each both at about 1/6 the size of Facebook.
I actually have an account with both of these.
At what point is the user base too small to matter?
1/6 of 1.28B is still over 200 Million people ... well at least created accounts.