They aren't perfect!
There still are problems with these networks however, and I am not asking you to signup for any of the above right now. The biggest annoyance is the crazily profuse amount of mails you'll suddenly start receiving, alerting you of friend requests, group events, and what nots! There are options to turn these off, but the initial reaction is that you're being spammed. The second is that random people will start adding you, and before you know it, your friends circle is full (if you fall for the noob mistake) of people who you don't want, tucking away the important people in a corner. Facebook does things right by actually asking you to estabilish relationship or a relating factor with the person you're trying to add. Once the other persons confirms that, he/she is added! These network sites should take steps to put barriers of similar kinds, so that you don't have a 1000 new 'friends' in a couple of days. Hi5 suffers from this, so beware!
Blogging can't keep circles together. They become specific, and is a good way to get to know 'new' people, and create your niche, get your thoughts across in a specific group. Hence, social networks are required. Its all part of the Web 2.0 boom (no longer a boom :P ), and like everything else, only a few companies got it right. Facebook got it right to quite an extent, by first putting restrictions on who could join, then opening up to everyone but again limiting to only certain types. For all these reasons, if you're looking to start with a network, I'll suggest giving Facebook a trial run.
What do you think about these social network sites? Are they really worth the time? Have they helped you in anyway? :)