That depends on the definition on "critical mass" :)

I'll say the growth of OzBargain has been pretty linear for the first few years. I guess there are some milestones.

• First million page view month: Sept 2008 (2 years after launch)
• First million sessions month: July 2009 (2 3/4 years after launch)

By looking at the traffic graph (from Google Analytics), I see a pretty flat line with some dips (Jan/Feb slow season). One significant event was at the beginning of 2011 when I go full time working on OzBargain. No, the traffic didn't shoot up (angle of the slope stays the same) but that just means I was more serious about growing it.

Looking back I realise that we are more "reactive" (adding resources when we have to) than proactive, but that's just me more used to slow and steady approach.