When facebook started, they were a 'closed' social network. An Invite only. You had to have a .EDU email address, and could only invite 10 of your friends. The Invite Only became an exclusive club and it grew quite quickly (virally). Then Facebook opened up.

Either way, you still need a seed round.

Traction is defined by Users or Revenue growth. In case of Facebook, users grew quickly and it is there where investors saw value. Facebook, Twitter, Foursquare and many others needed to raise money as their traction was defined by users and not $. Later on some of them where able to monetize users, finding that silver bullet.