One disadvantage is in such a system, once you publish something, you can't change your mind and delete it, any more so than you can with email.
At least if you say it while wearing a mask of pseudonymity or anonymity, you can shed the identity itself and grow another. And if you choose to defame homosexuals, people could look to see if you were wearing your corporate CEO hat or your religious zealot hat at the time--or even note that you do, in fact, consider them to be separate hats--before boycotting any chicken sandwich restaurants.
People do erect cubicle walls within their own psyche. Hardly anyone has a monolithic personality that is presented for all possible occasions. Many of the people who do have engaged in a lifelong pursuit of introspective remodeling to get there, as with a Buddhist monk who has trained himself to show equal respect to a homeless drunk as he shows to sober heads of state. Most everybody else would prefer to keep several different personas on file, to be used at need, including the persona that we show to no one at all.
The Facebook strategy to date is only serving the universal public persona. And that's really only suitable when no one on it actually knows anyone else. The instant any two pepole have an actual personal connection, the invasion of their privacy begins.
I am unclear why you write as if this is a good thing. I don't do business with people comfortable trumpeting how horrible they are just because they say "oh I'm only horrible when wearing this hat". I would rather that speech have consequences (particularly when someone is in a position to harm less-privileged groups).
There are legitimate reasons why someone would like to compartmentalize different areas of his life.
A doctor might be expected to wash his hands between every patient, but also choose to pick his nose before preparing his own dinner. When the superfluous lab coat comes off, my professional expectations of him as a physician end.
We have done this for centuries, with tokens of office and traditional garb. Kings have their crowns, Popes their grand miters. Judges don robes, and sometimes wigs. Cops wear badges.
Facebook does not allow for this. If you add your boss as a friend, he is connected to your everything. You have to work to separate him from your real friends. If, on the other hand, he were connected to your "corporate peon" role, you could switch him on and off at the... drop of a hat. In fact, he could give you that hat when you get hired, already connected to everyone else in your office. And if you leave that job, you both burn that hat, and he gives one just like it to the next guy.
It is a good thing.
sounds like good old corporate email/network/IM account.
It explodes beyond the ability of a human to maintain, and user names and passwords start getting reused.
I already have three separate e-mail addresses just at work, and that's a pain to deal with. I have two different personal e-mail accounts, so the recruiter spam doesn't drown out actual friend communication. I don't necessarily want Apple or Google or Microsoft connecting all those for their own nefarious purposes, but nor do I want to be burdened by the weight of my own requirements.