On December 24, social networking site Facebook saw its most traffic ever within the United States, according to new data from Hitwise. That tops Facebook’s previous record, which was set in July.

Facebook set a similar record on Christmas Day in the United Kingdom, and MySpace had unusually high traffic too. In Hitwise’s blog post reporting the numbers, analyst Heather Hopkins offers three possible explanations — crummy weather, boredom, and the urge to send holiday greetings to your friends. The last explanation was probably the main cause, she says, since Christmas Day last year also set a traffic record. The weather probably contributed too, since the cities with the most Facebook traffic (New York, Chicago, and Washington, D.C.) were all hit by snowstorms.

Hopkins’ explanation makes sense, but I’ll note that my Facebook profile has a real dearth of “Merry Christmas”-type messages, and I didn’t leave any messages of my own; I guess I’m a regular Scrooge. I’m curious about whether micro-blogging site Twitter saw a similar surge on New Year’s Eve, and early on New Year’s Day, when it seemed like everyone was leaving “Happy New Year!” messages. I’m tempted to ask, “Why the heck were people Tweeting when they should have been partying?” but that raises the question, “Why the heck was I checking Twitter at the same time?”

Update: Twitter co-founder Biz Stone tells us that the service saw about 1.5 times more tweets per second on New Year’s Eve than it normally does. He noted that while this was well above normal usage, it was not record-breaking-ly so.