"The best social network is still a table of friends."
I hated social networks like hi5 or facebook. In my twenties I didn't even had an account on any of these. Not Google Plus, not twitter, not Facebook, nothing. I had my friends close to me (the people I really needed to be happy) and I didn't see the need to have a part of my life exposed. The value proposition wasn't interesting enough.
Now, just a quick "side note": for those of you that are thinking "you only expose what you want", that's not entirely true. Someone can publish a photo with you, a bar, disco or restaurant. You name it. And this is just one way to identify you. Furthermore, as soon as a photo gets uploaded, you lost control of that information. Facebook or whatever now owns it. Even if you delete it, was it really erased from the repository? Or just flagged as deleted and marked as not visible? And how many people downloaded it before it was "made invisible"? You lost control. Just like a message you send using the chat or even a mobile phone text message. Another issue that I find annoying is that these social networks tend to use the word "friends" to describe the people you're linked to. I could bet that 99% of the people on these social networks have only 1% to 10% real friends amongst their "virtual social network friends".
So, back to the point, facebook was just something I didn't find useful. I had all the friends I needed (still have). In recent years, however, I've found a good usage for social networks: personal branding. Social networks like facebook turned into a personal branding media and they can be useful if you learn what to share, how, when, in what form, etc. Don't get me wrong, I'm not saying you shouldn't post "that lovely picture drunk on a saturday night". Just learn how to post it so it doesn't affect your image amongst people that are not your friends. Want an example? Suppose someone posts a public picture with you on a friday at 4a.m., having fun at a bar. Your co-workers see it. The meeting you had at 10a.m. goes wrong (and not because you had litle hours of sleep, but other factor whatsoever). Although there might be no relation at all between events, the fact is that your image can get hurt. So, my advice is simple: face these social networks as they're a program on television that anyone can turn on and see what's going on.
Here's an interview from Mark Cuban that illustrates what I'm saying:
As a final note, LinkedIn. I think LinkedIn is actually the best social network around because it tries to keep it professional. The advice here is actually that simple: keep it professional. Keep your LinkedIn updated with valuable information. It's the first "Google Search" that any employer will try to do when looking for your name: LinkedIn profile.