Early websites for social media: Facebook (2004), Twitter (2006), LinkedIn (2003) & MySpace (2003):pic.twitter.com/KQRY2PzOuy
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Wait but... why? 
Blue = trustworthy 
Pitch books are blue too.
I do miss the 2004 blue and 1-2px border look. I remember being so excited and re-skinning everything to go that way when I first discovered how to do it with divs: https://web.archive.org/web/20040122143948/http://www.omarbohsali.com/ …
A lot of us folks who are colorblind appreciate having websites that are heavy on the blues, blacks, dark gray, and (occasionally) dark green. Websites, graphics, and charts in yellow, red, light green, or light gray are a total pain in the ass.
Possibly because links used to be blue, and if there were a lot of links then the rest of the page had to work with that color. Blue seems fairly common in a number of older sites eg https://www.cnet.com/pictures/hotmail-through-the-ages-images/ … https://www.webfx.com/blog/web-design/popular-search-engines-in-the-90s-then-and-now/ …
Tho I’m not sure how common blue links were in early social media. Possibly blue had become a common color scheme for the links reason, and then that carried over a bit after links stopped being blue.
In some interview Mark Z. explains the reason of blue was that he, as a color blind person, sees blue color the most.
Blue was the color of the web 2.0. Does anyone remember the Flock browser?
Don't forget http://del.icio.us .
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