Welcome to Android Central! It's not simple to revert to an earlier version. On certain devices, flashing the previous ROM can be done relatively easily, although it does require a number of steps. Which device do you have?
If it's only Facebook causing problems, it may just be that you're running an out-of-date version. Go to Google Play Store and make sure Facebook isn't due for an update.
Facebook indeed has a new update on Google Play as of today. The description doesn't mention any fix/change for Lollipop, but it does include "improvements for speed and reliability." I'd say it's worth a try before attempting to revert to an older version of Android.