How come a lot of social networks/websites have a blue color theme?

[hi j]

For example Facebook, Twitter, Skype, Tumblr, Linkedin, old MySpace all have blue colored themes/logos. Why is blue such a popular color?

[Loliartist]

Probably because it is easier to look at and because blue holds attention longer.
It is the same reason fast food restaurants use colors such as red = makes people hungry
yellow = makes people assume quick
and some green = people assume healthy.