The answer is pretty surprising from this nicely done post by Ryan Healy of Brazen Careerist on his Employee Evolution blog. 

Here are some great data points from Ryan:

Twitter
The majority of Twitter users worldwide are 35 or older. Young adults 18-24 only make up 10.6% of the Twitter population in the US and are less likely than the average user to tweet. 45-54 year olds are actually 36 percent more likely than average to visit Twitter. (via Comscore) 

LinkedIn
The average age of a LinkedIn user is 40-years old. (via Techcrunch)

Facebook
The 35 to 54 Year old demographic grew at a rate of 276% over the last six months and the 55+ demographic grew more than 194% over the same time period, while 18-24 year olds only grew 20%.  (via iStrategyLabs)

Consumer Technology
A recent Accenture survey concluded that Baby boomers, defined in Accenture’s survey as those 45 years old or older, are embracing popular consumer technology applications nearly 20 times faster than younger generations. (via Accenture)

These are not just the toys of Gen Y.  Rather, they are being broadly adopted by Gen X and even Boomers in greater and greater numbers.  LinkedIn was once thought silly but now is a pretty mainstream business tool for recruiters, sales types, and super networkers.  Facebook didn't make a lot of sense for me (as my post on 8/29/2007 explains) until it gained critical mass from a population relevant to me. I now have connected/re-connected from folks across all stages of my life from grade school to college and beyond.

Most intriguing about the stats above to me is that the often debated role of all this in the enterprise (enterprise 2.0, etc.) is taking shape before our eyes.  The users of this stuff are (on the whole) adults with jobs who are gaining comfort using new technologies and are adopting them to both their personal and professional lives.