I'm rather specific with regard to social media. I've never been one to do something because "everybody else is doing it", which is why I'm absent from websites such as Hyves and Facebook (partially due to the barf people turn their profile pages into, and the fact that I do not consider a cloud of smiley faces to be communication - yes Hyves users I'm looking at you).
On the other hand, I do have a profile on LinkedIn (which I update once a month tops), and am an active user of Twitter. The latter is something I find extremely useful. People tend to get really creative when limited to 140 characters. However, there are a number of things I find extremely annoying, and will often result in me unfollowing you.
Automated Direct Messages
I enjoy having conversations with people I follow. I might not respond to everyone, or every tweet I find interesting, but I really expect Twitter communication to be two-way on some level. Sending me an automated message telling me "Thanks for following me" is something I interpret as you considering yourself more important than your followers (either send them a personalized response or don't send them anything - most people don't send anything so it's acceptable) and my usual response to automated welcomes is a Unfollow.
I like following people who have something interesting to say. If all you ever do is Retweet other people's stuff you look like an automated drone. Not exactly my primary candidate for following.
Deluge of people followed
Some people follow 20K people. There's no way in hell you can keep up with all that. Such a person following you is essentially meaningless as they'll never notice any of your tweets, and they will therefore not respond to them (which brings me back to the two-way communication part).
Some friends of mine like to Tweet every cup of coffee they drink (literally). However, they post about 10 tweets a day on average. This I can more or less keep up with (thanks to some heavy list usage voodoo), but once you consider I follow about 200 people, it gets a bit crowded when they all start tweeting at this frequency. As such, I am rather reluctant to follow people who "tweet aloud" - that is, share every minute detail of their life.
That said, Twitter is a tool I enjoy using despite having a few pet peeves. What are your Twitter pet peeves?