In fact, this would actually force people to block FB. The worst thing to happen would be for someone to post important information related to company on their personal feed by mistake. If they can login to both the accounts, this is very much possible. Unless, of course, FB for work looks dramatically different from original FB. I don't know, I never tried.
I work for a Large Corp., and FB is not blocked. What is blocked are services like dropbox or google drive. But I can't imagine the security department would allow publishing work-related information to other company cloud.
The business functionality has already existed (since FB uses its own platform internally), so its comparatively minor in engineering effort.