I think almost everyone under the age of 30 probably has a Facebook or MySpace account these days, especially but certainly not limited to singles. My impression is that the newer Facebook is catching up quickly and may be more popular than MySpace in Canada; I suspect MySpace still has the edge in the US.
facebook is really an online social networking tool. you sign up, you can search for old friends, or current friends, and add them to your friends list. you can upload photos and share just about anything you want to.
it's really quite neat and fun to find long lost friends; i've reconnected with some that normally i wouldn't have. the flip side is sometimes people want to add you as their friend when they're more of an acquaintance and you might not want to reconnect with them.
the other thing, both good and bad, is that you can see who's friends with who (if allowed), and you get a good view of how people are linked to each other. this is interesting if you're the one doing the looking, but it might not be so great if you're the one being looked at (reduced privacy).
i've seen some my space pages and not really cared much for it, but facebook seems to be a bit more organized.
as with everything online you need to ensure you set your security settings appropriately. facebook has been getting a bit of controversy as it's doing things like letting google view pages; but i think that is something that can be easily fixed in your settings. the trouble is most people aren't aware of how open their profiles are and it does take a bit more of a seasoned internet user to be aware and to change the defaults.
Generally, I conscientiously avoid these types of sites. Unfortunately, one of my friends from school is going overseas for a year and has stated that she'll only keep in contact via Facebook... hence my decision to make an account. I'll be using it with great reservation.
I registered with Facebook to keep in touch with my sons (one of them sent me the initial invitation) and with nieces and nephews. I'm not on there very much but you can get notifications about "status" ("John Doe is off to Autralia for a vacation", "Jane Doe is excited to be starting her new job") delivered to an RSS reader and/or via email so I get some family news even if I don't sign in.
I think Facebook is the better choice. Myspace seems to be a haven for paedophiles, but Facebook allows you to decide who can see your profile. I have a facebook account (I think someone else created it for me), but I would never get a myspace account. I have a public blog as well, and I prefer facebook for more personal posts with personal information, because the only people who can read it are people I know and trust and allow to read it. Myspace is a bit different -- everyone can read myspace, and that worries me a bit too much for me to ever feel comfortable using it.
One of the goals for social networking sites in general is to provide a venue to link together people with similar interests.
For example, another psychology blogger has established a group on Facebook to foster communication among people who publish psychology blogs. I'm not very involved in it and that's just one example but it may be that an organization would want to participate in or subscribe to such groups as a way of advertising their existence or presence to a target audience.
I'm not sure which organization you mean, Steve, but as another example there may well be a Tourette's group on Facebook, in which case an organization such as the Tourette Syndrome Foundation of Canada might make a decision to establish a Facebook account with some information about their organization and join the Tourette's group. When someone new joins the group, one option is to scan information (or what information has been designated by the owner as publicly available) of other members of that group and the Foundation would then show up as one of those members.
BTW, Craigslist is a good example of the dark side of social networking, IMHO. It allows cheating spouses, typically husbands, to post ads asking for anonymous sexual hookups. And then there was this development, where an innocent woman used Craigslist to find nanny/babysitting jobs: