What's interesting is how little thought seems to go into differentiating web sites through colour palette.
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.