It has been HEAVILY speculated (nearly proven) first by incredible users and now by some of us EVO users that facebook for HTC is one of the root causes of poor battery life. A few incredible users suggested that it had a bug that made it update constantly no matter if you had sync on or off and no matter what you set the sync interval to, the only way to stop it was to either root or do a factory reset and not re-enable it. After doing a factory reset and not re-enabling it my battery life has more than doubled. I'm currently at 8hr 43m unplugged with 62% remaining after using the phone semi-heavy through the whole day. Before the factory reset the phone would get to 0% in about the same time frame.
There is debate on whether or not it is the factory reset or the not re-enabling FB for HTC that is helping battery life, but all I know is that 12+ more hrs of battery is more worth it to me than having my FB contacts synced and a live wall feed so I'm not taking the risk.