Here's a new word to scare you: Faceniff
Funny word, scary possibilities. It's an Android app that lets users intercept web sessions on wi-fi networks, steal credentials of people using Facebook or Twitter or other sites, and then impersonate that person. It's apparently a lot easier to use than Firesheep, the last big such program to scare us. It looks like Faceniff can also work on a WPA encrypted network.
Now, can hijacking your Facebook or Twitter cause you instant financial ruin and cough up your social security number? No. But if someone can start posting as you, there are obviously a lot of potential problems for your relationships and trustworthiness.