I am hesitant to say that people shouldn't be able to vent about their work on their personal facebook page, especially if their personal page isn't linked to their business page. That said, if it's something you wouldn't want repeated, you really shouldn't post it online.
However, just as I may vent about work at home or with friends, there may be times when someone might be outspoken online looking for advice or encouragement. At what point do you judge who is "bashing" and who is encouraging conversation? Would you insist on reading the private journal of an applicant? Would you not hire a candidate who refused to grant access and has their privacy settings to allow only friends to view? Do you really need that level of personal information? And, despite your use of quotes in 'personal'... it is PERSONAL. As long as it isn't linked to business, and you don't have people you do business with (vendors, coworkers, residents, etc) as friends, it should be viewed as personal property.