4Chan is only 4 months older than Facebook and is hardly that profitable. Which is what changed, online dumpster fires where not attractive to advertisers but add a veneer of social networks and some basic location / demographic data and suddenly things change.
Facebook really had two periods because it was limited scale prior to mid-2006, when the number of users grew dramatically.
I think the more important point is really the advertising angle: taking a bunch of small communities and giving them high-quality hosting powered by a billion dollar company along with a pre-developed audience.
