I started this off as a post on Flixster, and why they're a great example of a solid niche social network. They focus extensively on movies and have received a ton of press. It's a simple and fun site to use.
As I was chicking out their tipping point which seemed to appear around December 2006, I began to question if other sites may have a similar tipping point, perhaps more pronounced.
This is Digg's 3 year:
It's easy to spot this one. Wow. This is when they released a new version.
Now look at digg's competitor, reddit:
It's almost the same tipping point.
Flickr's looks similar and even more extreme:
What causes these tipping points? My guess looking at the examples above are a new version, the blogosphere, adding viral tools such as Digg did in February 2006, or getting purchased, such as Flick above.
It certainly can't be traditional marketing. Whatever it is, it has a great and lasting effect on traffic. There doesn't seem to be a "crossing the chasm" portion to web apps, or at least these succesful web apps have made it appear that way.