If you are JUST using it for syncing contacts, then the launcher you use shouldn't matter. The sync process is handled by the OS in the background. You just won't be able to use any HTC widgets. I use LauncherPro on my personal EVO and can validate that the HTC Sense Facebook will, in fact, still sync.
Now, please forgive me for asking the basic questions here:
Have you double checked the sync settings? I know, this is an obvious question, but can be easily overlooked. Make sure under Accounts and Sync that first, the little circle icon next to Facebook for HTC Sense is green. If it isn't, tap on it, and check the Sync Contacts box. Also, while you're there, check the Sync Frequency under Account Settings.
Next, if you just want to sync contacts, you might give the
real Facebook app a try. It can sync contacts just fine, again, regardless of the launcher you're using. If you do go this route, a word of caution though: Don't allow BOTH the real Facebook app and the HTC Facebook app to sync contacts. If you do, you're going to end up with lots of duplicates. Just go back in to Accounts and Sync and disable the contact syncing for the HTC Facebook app.