It's a common consideration in other industries to establish a clear colour choice as a method of differentiation. Take Coke red, for example, versus Pepsi blue. Or green John Deere tractors or yellow Dewalt power tools. Any new entrant would be wise to pick a specific colour and stick to it.
Looking at the colour maps here, were I to start a social network, I would go for a scheme in the yellow/orange/red spectrum to separate myself from the dogpile of sites lumped in the blue area. Likewise, for a blogging host, something in the greens or red/purples would be good.
It looks like the location and social news sites are doing a better job of differentiating themselves.
Also take a look at Google and Microsoft, clearly "rainbow" companies. Apple used to be a rainbow company but has done an excellent job of shifting their palette over to the monochromatics. Not only is it as far away from rainbow as you can get, but the monochromatics are associated with "expensive","premium" and "well designed".
I think rainbow palettes actually kind of backfire when it comes to big companies. It's like they try too hard to say "hey look at us, we're so fun!" while also looking unfocused. However, overemphasis on one colour can backfire too.. just look at Yahoo and all their purple crap. They just seem completely dorky.
If I'm starting a social media company though, I probably wouldn't want to venture out too far away from blue - simply because it's the easiest way for users to associate it as a mainstream social media company, i.e. alongside the Facebook-Twitter-Linkedin-Foursquare league, unless you are competing head on with any of these big four. I know this is a cheap strategy, but it also makes it cheaper for you to establish yourself in the market.
Or, just pick your favorite color, much easier and you'll always be comfortable with.
Is it good to be different merely to "separate yourself"? You could go with having white text on a black background to differentiate yourself too, but that wouldn't necessarily be a positive..
Promiscuous Blue would be a great name for a band.
Random trivia: In order to combat the fact that clothing gradually turns yellow as fibres are damaged, modern laundry detergents contain dyes which fluoresce blue under ultraviolet light. The "bluey whiteness" really is exactly that!
Blue dye != blue fluorescent dye. Blue dye simply absorbs non-blue light; blue fluorescent dye absorbs UV and emits blue light.
International Klein Blue is probably the most promiscuous blue you can find, you cant reproduce that unnerving vibrancy on screens yet, pity.
Wow. That is some blue blue. And that's not even it.
Apparently it's the same blue used by the Blue Man Group.
(At least with the designers of the icons for the applications I use most often.)
From experience, it's also much easier to combine red and blue with a grayscale palette (iTunes icon, twitter to name two), which may also contribute to their popularity.
As for green (money, health, fresh)
It has articles, tools, a bibliography, and an entire section called "Designing with Blue":
I didn't realize they are still a force. Didn't facebook solve this problem?