Online, I don't use it so much, but I'm sure you'll find a post with me saying "uh..." somewhere around. Offline is a different story. XD; I guess it's just a bad habit, like if someone asks me a question all of a sudden I'd pause with an "uh..." and then answer them. I try to avoid doing so, but lately I find myself using, "hmm" rather than "uh..."
Most of the time though, I just get mocked at from my parents since when they usually ask me a question I say, "Uh...I don't know." My grandpa used to pick on me about saying "I don't know" a lot as well. Bad habit I guess, but maybe I'll try to work on it. D: