That is a real concern. Already, my firm rejects candidates if they don't pass the social media test. For example, if you posted pictures on Facebook of yourself at a frat party using a beer bong John Belushi style, chances are, you will not get hired. Or even posting something controversial.
I will likely retire from the job I have presently but I am fully aware that should I seek employment elsewhere, and my Free Republic handle is matched to my real identity, no company will hire me due to my conservative beliefs.
It's a real concern and it's going to get worse.
And if you do not have a Facebook profile, you will not get hired. Some corporations are already doing that.