"Doing single" is a term most people with social media know. It means that you can post a picture saying, "I'm single, date me" on like snapchat or something, or if someone asks you if you are single, you can say yes. If you are currently talking to someone, but you aren't cuffed, the person you're talking to might get angry if you're still doing single.
Joe: damn sexy , you single?
Stephanie: yupp
(Stephanie is doing single.)