An account on a social network dedicated to the steamier or more controversial side of a better-known blogger/tweeter/poster. As opposed to one's public ("Daytime") accounts, AfterDark accounts are usually non-public, NSFW, and require a request to be added before these are visible. This keeps the content out of the boss' / public's / spouse's / lover's eye. The name alludes to what goes on outside of work, traditionally at night, such as sex or partying. Some AfterDark accounts include the words "AfterDark" in the username, while others use different or obscure names so that the true owner will not recognizable to strangers, but clear to anyone who knows him/her.
Some believe the name is also a reference to the After Dark screensaver, an early computer program of the same name introduced in 1989, that displayed moving images to prevent a static image from burning dark spots into the phospor of screen display tubes during idle times (nowadays, the term "screen saver" makes no sense for LED or LED-backlit displays).
His afterdark Twitter profile says "The private life of a certain spotted cat. Very NSFW. If you know who this is, and I know you, DM me and I'll add you."