Twitter - game dev related stuff, links to interesting sites, articles etc.
YouTube - compilation videos (235 Free Indie Games in 10 Minutes etc.) and game related playlists (platformers, shmups, playthrough collections…)
Facebook - links to interesting sites, articles etc.
Tumblr - game related discoveries from around the net
Pinterest - a collection of video game related boards (indie games mega list, games that were created with gamemaker/unity/udk… pixelprospector art gallery, greenlight picks…)