Twitter overwhelmed me for those reasons--I'm REALLY not good at having an online personality. I'm a lurker at heart, and a homebody in real life, but I pretty much stay away from society in all forms, digital or otherwise, unless I'm at work (DIRECT EVIDENCE, I'M A PRETTY ACTIVE-ISH MEMBER HERE BUT I HAVE LIKE 200 POSTS LOL). I can turn myself "on," I just can't do it online, which meant Twitter for me was like watching everyone speak a different language, get cookies and prizes for it, but I don't know how they're getting their cookies and prizes because I don't understand anything that's going on. It's as everyone has said, just like high school, but even in high school everyone eventually finds their own clique.
If you find your confidence uncomfortably low, do what thejadevoice (and myself) have done and get rid of the space (or distance yourself) that is causing your doubt. Your world won't end without you being on twitter consistently, just tweetdeck some and pre-tweet, check in for some user engagement, and blast away. I have said on some other post, long time ago, that my dream has always been to be a reclusive writer, but even I feel the draw of the exclusiveness of some of the big name author circles. It's only natural to feel left out when they're your contemporaries, which is why you should disengage if it effects your process, and what you like about the craft.
Actually, I just joined twitter, and I avoid it. I did because I sense that if you want to be an author in this day and age, you "have" to do it. But I'm never on there, and there doesn't seem to be any point. I'm not much of a "have to" person anyway. I pretty much stopped going on facebook about nine months ago. It ended for me when a woman I'd just met came to our house for a drink... she grabbed her phone in the middle of a conversation and said: "I just have to see how many likes I've got on my last facebook post. The pictures of my pets are so cute, it's going to be a hit." When her post "only" had 14 likes she was mortified. Oops. When our lives are governed by how much online attention we're getting or not getting, then something in our society has gone horribly awry.