It seems to reveal some, but not others. A friend of mine at work picked up on Twitter within the past week, and his profile is private. His first tweet shows

It seems to reveal some, but not others. A friend of mine at work picked up on Twitter within the past week, and his profile is private. His first tweet shows up, but no others, though I can see a few his friends have retweeted his messages. I do not have a twitter account, so I was not signed in.