These friends feel that Facebook is not really usefully, too diffuse, and most of the applications on it are a waste of time, which means it is great if you want to waste time, but not as a tool. Other friends (and sometimes the same friends) are also not hot on LinkedIn. They like it better, because it has great critical mass in the job field, but they're frustrated by the clunkiness of the search, the cumbersome communications tools, and what they see as the employment-specific narrowness of the experience.
I'm an active FB and LinkedIn user, and I find them to be invaluable, but that's because I use them as explicit directories. In other words, when I meet people I'd like to stay connected to (and remember how to contact them and who they are) I add them to FB and Linked In.
LinkedIn works as a professional rolodex for me, a list of people I am connected to that allows me to see what they are doing and have done and how to reach them.
FB also keeps me connected, but that's the color wash on the black and white LN listing--if it's someone I've met briefly, FB can bring them to life as a more 3-D person--someone who updates their news feed, likes particular kinds of sports and music, and--often--shares friends with me.
Basically, what this means is that I use these two services to create explicit social network directories--listings and contact management that provide useful and interesting records of people I meet.
Interestingly, one could argue--and I will--that the biggest wasted effort is email. Your email list--particularly the people you message, as opposed to the people (and spambots) that message you--is a rich contact list, but the data attached to each person is pretty much buried. There's no way to know that email@example.com is me, and to assemble the meta data about who I am that will provide as rich a picture (and not feel totally stalkerly) as LinkedIn and Facebook can.
(And yes, I know people have been talking--and trying--for years now--to build email out as the basis for a social network, and that *smart* email is one of the so called Next Big Things--just show me the products and the people using them, folks).