Older people just don't give a shit about what Facebook, Twitter, etc are doing. Facebook has recruited me heavily the but I have no interest in anything they do. I am not going to uproot my family to work for FB, Twitter, etc.
At an old 33, I prefer to work on either my own company or optionally if that wasn't there, companies which share my passions.
Young People are great to hire because they'll work their fingers to the bones for some abstract promise, but once you build a family and a life, working 80 hours a week, because startup, isn't appealing.