Our traffic is spiky, but not as much as the Alexa graph suggests. In reality our growth is much more steady; every now and then Alexa seems to "catch up". It's weird, and we're not sure what would cause that effect.
Just so you don't get the idea I'm picking on Alexa, estimating this stuff is hard (or the people doing it are incompetent, but that seems unlikely with so many entrants). Quantcast and Compete.com are even worse, and ComScore is expensive but no better than Alexa.