Think about it as concentric circles. Facebook is the tightest circle—people you largely actually know, or interact with on a personal level. Twitter is one more out—people you're aware of, and whom you like, some of them you know, some you don't. If those networks are like the orbits of Earth and Mars, Digg of old, and Reddit are like the orbit of Pluto.

Facebook and Twitter did not kill Digg. Ask anyone who used to frequent the site. Where to find this person? Why, on Reddit of course. If Digg had had their botched v4 launch before Reddit was a legitimate competitor, they might have had enough time to clean up the mess and get the site back on track. But instead, they launched a product that betrayed their community, who readily moved over to Reddit, who largely welcomed them with open arms.

Digg'ing your own grave http://www.pixelmonkey.org/2012/07/13/digging-your-own-grave

Reddit (http://reddit.com)

Boxnutt (http://boxnutt.com)

Snipit (http://snip.it)

What others have you guys came across?

http://thetechblock.com