Twitter Worm(s): 190 accounts, 10,000 tweets;
Samy Worm: 1 million accounts

I think Samy takes this one, especially since it was first.

Also important to remember that these social networking worms "infect" profiles or pages and propagate on a controlled set of hardware. Damage is typically very limited.

Not sure whether/why people might suggest this was significant, except as proof of concept. But we knew it was possible, right