I'm going to go with the first thing that popped into my head: Facebook.
Don't get me wrong, Facebook has its good qualities. It's a good way to keep in touch with friends, but you know what? So many of the people we "friend" are the ones we don't really care to talk to, write to, or ever see again. We friend them because ignoring them will only make them mad or offended. Also, I think Facebook has kind of spoiled us. We can know anything about anyone at the click of a mouse. Then we post pictures of ourselves and our loved ones for our friends to see, but anyone who's smart enough can find a way to look at them, too. And no one really thinks about the consequences to some of the information they decide to share. Employeers can look at your profile pages, and if your page is public and a major portion of your status updates are about partying and drinking, then that might cost you a job.
I do love LiveJournal, I'm starting to really love Tumblr, and I like Twitter, and I like Facebook enough to use it to keep in touch with some college friends. But at times, I do think the world might be a little better without Facebook. It's one of them double-edged swords. The world was fine before it was ever invented, but now it feels like the world can't live without it.