Found one of the results how Facebook expands their platform so fast.

I am wondering what happened on livedoor Co. Ltd., when they finished livedoor Mail, if they allowed users to export all the email list to their original SNSs at the time. They have still better user-oriented platform rather than now. To delete, or not to delete. To connect partners and competitors, or not connect. It’s been a long time ago.
Anyway, Facebook has a white list of 720 domain names to access their mail services and their mail accounts lists to ease to connect there. At least, they can keep accepting 2.2 billion accounts* from Yahoo! Mail, Windows Live Hotmail, Google Gmail and AOL Email every single day.  *http://bit.ly/jl3dG