Last week, Twitter reached a milestone by hosting its 10 billionth tweet! Unfortunately, the world will never know what the tweet said, since the account holder keeps her account on the down low — as in, private. I'm very curious to see what this person was tweeting about, just like I was super excited to learn what the 10 billionth song download was on iTunes. Like Facebook, some people only want to let their closest friends in on their thoughts and info, but tell me — do you keep your Twitter account private?