Well, I have been in the social scene for a 3-4 years now. I was using MySpace when it was BIGGER than Facebook, back with its first original profile (when everyone in the world loved it)
To people saying MySpace sucked, it really didn't suck. It had so much on Facebook, things people loved. The music player, custom backgrounds and themes for YOUR page, etc.. PEOPLE (mainly teens) loved these. When MySpace took them away, it killed it...forever. How do I know this? Because people talk about it on Facebook or even face to face like me and other friends. Teens LOVE stuff like this.
After realizing that NO social site will ever have the features MySpace did, people had to choose: Facebook or MySpace (fast and simple [[Facebook]] verus slow and complex [[MySpace - their new design/layout]])
To those talking about timing, I don't think that has anything to do with it. I believe social networks grow by targeting younger people first. More teens and younger people use the internet for friends and chatting. How often do you go into Yahoo! chat rooms or AIM rooms and find middle aged people? Not very often, usually younger aged people.
Facebook was smart and realized that also, teens/younger people (this can also be applied to any age really) LOVE reading about their friends and what others are doing, because in high school you also wanted to know who was fucking who, etc.. So Facebook made the Activity feed that showed what ALL of your friends were doing.
Now, once thousands of teens signed up.. Parents started to wonder about it. Remember when Facebook first came out and all the bullying was going on and kids getting killed because of Facebook? I believe that once parents found out about this, they joined to "watch" or "monitor" their child. I see my friends parents in the "Suggested Friends" all the time, which is honestly quiet weird. So now, you got a bunch of teens with parents all joining. Now parents realize they can keep in touch with old friends so they all start spreading the word, etc..
Now, I don't see "Ghetto" or "Hoodlums" using Google+ just because it doesn't look like a site that any of them would go on. Facebook is just a plain page with a logo and text, it will fit anyone. MySpace was so popular because people could make their page ANYTHING... If I wanted a weed background, I had it. If I wanted to put my favorite songs on my page, I could.. PEOPLE LOVE THIS SHIT. Facebook is just a way to stalk your friends which is why everyone will use it. Also, Facebook is a huge cause of break-ups / drama in high school so everyone also uses it to keep up with news/rumors/etc.
For a new social site to come up and take the scene, they would need all of this to overcome Facebook..
Thats my look on it. :P
EDIT: Also, to kinda prove my point.. Look at Facebook, it started with college kids (younger aged people
)