I'm going through this now for the 2nd time this year, the first time Facebook kept me from posting from my Macbook Pro for 3 weeks. The 1st time, I ran Sophos and it came up clean so I did nothing, they just let me in one day. My iMac, iPad & iPhone each still had access.
Today: I ran Sophos earlier and came up with "no threats found" as well as "issues detected", came to this community and found several posts stating that "issues detected" is nothing to be concerned about, just Sophos finding encrypted files (same with a Google search). I decided to also download ClamXav from the App store and am running it now... 21 Win Trojans detected, so far. All related to Ebay and selling on Ebay. Thought I had cleaned up the computer good, as it is very limited on space and I'm not upgrading until next May. I'll contact FB tomorrow to clear this up, don't contact through their help center, Google best way to contact facebook and you get more direct options.
Bottom Line: ClamXav - download it Free from the App store.