I think the "only" part is where you could get into trouble. That is limiting your pool and possibly bydisparate impact on one or more protected classes.
I don't know of cases, but would suspect that any claim would be under age discrimination under Title VII and ADEA. This generally states that "Any action that an employer takes that adversely affects a disproportionate number of employees over 40 is also age discrimination."
The question would then become does posting to social media ONLY adversely affect older applicants? Personally being just over 40 and not in a highly technical job, I would say yes. However, I am not sure it would be enough to rise to a point of a court case. I suspect most employers would settle prior to getting to a court case on something like this. I am not sure there would have been enough time to get to a court case as just now the courts seem to be addressing what rights employers have over employees online presence/account/etc. So I wouldn't wait for a court case to tell me it wasn't the best business/HR practice.
Another article I found stated "Revised Age Discrimination Regulations: ...Under the new regulation, practices or policies that have a disparate impact on workers who are 40 and over will be considered discriminatory unless based on a "reasonable factor other than age." According to the EEOC, an employment practice will pass muster when it is reasonably designed to achieve a legitimate business purpose in light of the circumstances, including its potential harm to older workers.
The number of age discrimination claims filed with the EEOC has increased by 50% since 2000. The regulation is a response to concerns that older workers were being unfairly targeted as more employers downsized in recent years. The ADEA applies to all private employers with 20 or more employees, state and local government employers, labor organizations and employment agencies."
One last article can be found at : blogs.payscale.com/compensation/2011/07/eeoc.html