What will be the second site to hit that landmark, does it even exist yet?
GMail ~425M Hotmail ~350M Yahoo ~300M
(http://www.email-marketing-reports.com/metrics/email-statist...)
http://en.wikipedia.org/wiki/Tencent
"As of December 31, 2010, there were 647.6 million active Tencent QQ IM user accounts,[1] making Tencent QQ the world's largest online community. The number of simultaneously online QQ accounts has sometimes exceeded 100 million.[7]"
Definitely beats gmail's 425 million.
Why do you think that (seriously asking)? Gmail didn't become available to the public until 3 years after Facebook launched (admittedly Facebook was initially limited to certain groups of people). Before I signed up to Facebook Hotmail was still very popular with everyone I knew, mainly because they all used MSN Messenger. It's only been in the last 2/3 years or so I've seen most of the people I know default to Google when creating an email account. Up until then Hotmail was all people really new.
Facebook (or rather, TheFacebook) launched just a few months before at Harvard.
----
[1]: http://www.quora.com/How-many-unique-visitors-are-there-to-Y...