What's wrong with something like this, for example? http://www.jobvent.com/
Jobvent looks like more-or-less the same basic idea.
One issue with translating this idea to startups is that if it's a small enough startup, they might be able to tell who you are. You might be the only person who's ever quit that startup, or there might be few enough that they can tell anyway. This could cause issues where companies threaten to give bad recommendations to future employers if people use the site, or something.
So if a boss has high turnover rates under him, the idea would be that the reason why could float on his Unvarnished profile, such that would-be employees can get this information from the market before signing up to work for him.
Awesome hiring managers should get credit for being awesome. Bad hiring managers should not be able to hide behind information asymmetries. So too with employees.
Information liquidity is a powerful thing!
1. Your basic value proposition rests on getting people to pay money to have the ability to respond to things said about them. That's going to leave a bad taste in people's mouths.
2. Many people (like me) have deliberately chosen to not join facebook. We can't even try to address issues we see.
3. Identity is much more complicated than you acknowledge. My name is Ben Tilly. I'm at Google. People by the same name can also be found in a US college, doing graphic design in New Zealand, and being a nurse in England. Just to name a few that come up off of the top of my head. How do you tell which Ben Tilly should be able to respond to a bad comment about someone named Ben Tilly at Rent.com?
4. Anonymity is not so easy to protect. Only a small fraction of people comment online. A given manager only manages a small number of people. If a negative comment appears, frequently it is surprisingly easy to make an educated guess. If the comment is at all detailed, an analysis of writing style will usually confirm that guess. Unless a large fraction of people in your environment are commenting, the level of anonymity is limited. (Particularly if the boss can get access to proxy logs saying who was visiting getunvarnished, when.)
I wouldn't call this "can't". If you actually couldn't use their preferred identity provider, that would be different (and it could happen; I had a lot of trouble getting a Facebook account at all a few years back, due to my unusual name). But choosing not to sign up is not the same as not being able to sign up.
The unintended consequence, of course, is that people who don't have a Facebook account can't just create one. Instead you have to create one, use it until you have a sufficiently well-established identity, and only then can you use their service. Which is a high barrier for people who actually don't have a Facebook account.
Highly unlikely considering what you have created. Anonymity and credibility are mutually exclusive. What you have here is a libel engine.