As a follow-up to the top searches of 2009, here's what Twitter says were its top subjects: Iran, Google Wave, and Michael Jackson (that's a TechCrunch blog post; see also the original Twitter blog post).
Twitter broke the topics into categories, so the Iran elections were the top news story as well as one of the most popular hashtags. The world really is getting smaller when a new piece of technology can spread the word about an event globally within minutes. Why Google Wave? No idea. Michael Jackson was a worldwide icon, so it makes sense that his death was huge news. Harry Potter and American Idol were also popular. Again, no surprise.
TechCrunch speculates about items that did not appear to be as popular, such as Facebook or the iPhone. But why would you tweet about things that are already part of the zeitgeist? Facebook provides its own communication avenue with status updates, and the iPhone is for calls and all kinds of other stuff through the apps. Unless something drastic happened, like a severe outage, I don't think people would need to tweet about Facebook or the iPhone.
I did think it was interesting that Tiger Woods and Lost were on their top tens lists. Lost hasn't been on the air in months and won't be back for a couple more, but it still has staying power. And most Tiger news didn't break until just a few weeks ago, so the impact of his crashing and burning seems to have pretty large.