networking with little or no real networking?

i am a member of a few networking sites, primarily Linkedin and Facebook. the others i have registered on are xing, marzar and maybe a couple more here and there. my reasons have been well defined in my mind- facebook for my personal network and linkedin for professional networking. i'll even stake a bet and say that these are probably inline with most people's perceptions of the respective networks, including the networks themselves.

i've found that on either site, particularly linkedin, people are adding numerous contacts to their list all the time, while there is little actual networking. i can prove it- i have at least 10 contacts on my high school list on facebook, none of whom i ever speak to, and i have empirically proven it. 80% of the time when i msg some of these people, i don't receive a response.

on linkedin, i have had a few experiences where people either simply do not respond, or get irritated when you ask them to help you with something or seek an introduction. this despite the fact that people call themselves LIONs or open networkers. on the other hand, the hypocrisy lies in the fact that they rarely if ever examine their own barrage of Q&A, free advice or adding people at random, just so they can be placed on an irrelevant mailing list.

i would appreciate any insight on why bother even posting a profile when you're never going to use it! is it just a numbers game? perhaps an ego issue where you can get a leg up just by showing off your 10,000+ contacts?