But employers who turn their noses up at Facebook et al. may well discover that their coveted Millennials (a.k.a. Generation Y, a.k.a. those damn kids who won't get off your lawn) are happy to return the favour when recruiting time rolls around. Blocking access to Facebook looks a lot like those IT departments that wouldn't install web browsers on your computer a decade ago... or external email access a few years earlier.
And like those tools before them, the social web today is increasingly being used by companies and organizations for productive, collaborative work. So it's not just a question of denying your HR department a hiring pool of cool kids. Blocking social media from your company can mean cutting yourself off from an important potential source of productivity, innovation and increased efficiency.
Of course, that's an argument I like to make to people who haven't just received a dozen Farmville notifications.