I used to be on a lot of social networks years ago... as in 10 or so (when I just had internet for a few years). Mostly dutch social networks and spent some time there. Back then, pretty much like I still am now, I'd just fill in a profile of sorts, perhaps do some graphic layout and if someone liked what they saw, they'd sent me a message or a friendrequest. I rarely do friendrequests myself because I feel that I should have to offer something and I'm not really sure what asset I might be to some in their social group. Unless it's people I already know from somewhere (some people from this board come to mind).
Over time there were some international social networks around like Myspace, and later on now Facebook. Was on both, now I'm still on facebook. Never was on LinkedIn, though I had some requests from that in the past. Oh... and then there's a few of subculture social networks. Vampirefreaks comes to mind, I'm still on it somewhere but I'm not really active. Last time I really was active was about 2 years ago, when I did my last graphic profile overhaul. And recently I got on DeviantArt, which is a fine network I think if you're into arts, either creating it or just browsing pretty pictures and the like.
As for Facebook and it's pro's and con's... or any social network. I wouldn't now... it's not as if I really, really, really care what all my friends are up to. Yes, I do respond to some things, but in general I do like the funny anekdotal one-liners people come up with over the "I just bought a new couch but it doesn't match the rest of my furniture"... I don't know why I should care for such trivialities. I'm there for fun, not for some heavy ******** lifestories. So that might be the biggest con I have in general with a lot of social networks, some people just pollute my newsfeed with their bad news (or sheer stupidity). At some point those people will be either blocked from my newsfeed or unfriended. The game requests itself, I don't care for. I don't play any and I rarely get any requests either. I used to get a few when it was new and all. Not having a lot of friends there helps, because if I were to add say... 5 to 10 people a day, chances are people will eventually send me an invite. Ever since I'm on FB (about 2 years or so now) I have just under 70 contacts and half of them sent me a gamerequest (in the span of those 2 years)... that's in general what I think is manageable.
Any pro's on social networks? Sharing fun stuff with people so you can both have a laugh, that to me is the best thing about facebook. Well, and to some extent, having some bands/artists in the newsfeed keeps you updated on their recent happenings as well. I rather do it over facebook, than say... visit each and every one of the sites daily/weekly.
Oh.. the layout thing.. I'm happy I'm not having Timeline pushed down my throat yet. If that happens (probably soon) I'll look into how I can make changes suitable for my needs (privacy and all) and if that doesn't work out and I can't get used to it, I'm out.