Today's lesson is brief, and perhaps speaks for itself, but may be worth covering just in case.
"Follow back" is a social media term used to discuss reciprocity. It might be used as a request ("Follow back?") when one person follows and/or friends another, to ask if that person will follow or friend the first user. (For example, I might add you to Twitter and ask or expect you to follow me in response.)
Or, a person might add to their bio online, "I do follow backs." That person is saying, "When you add me, I add you, too!"
Accumulating friends and followers is important in social networking (unless you are cranky, like me), and particularly important to Young People. A high number of followers may indicate popularity. (As an Old Person, you have every right to scoff, "Right! But do you know those people?!" See, I'm crabby.)
Some accounts—for most sites—exist for this purpose! Personally, I think these accounts as just as bad as spam robots. Some spammers create links, or fake websites, luring users to follow their accounts in return for an exponential increase in followers.
There is also a level of guilt associated with this action, though I doubt it is served in large doses. I found an abbreviation for this phenomenon, ISFBG (I Should Follow Back Guilt). I can't confirm that it's a popular string of letters, but it's worth knowing, I suppose, if you come across it (or suffer from it). Having too many strangers friend you, and pressure from those strangers, can occasionally needle the nerves (and patience) of the user. So, you know, beware?
The request/phrase is also sometimes shortened to "follback." If someone asks you, "Follback?" you have my permission to laugh uproariously at this stupidity. Seriously.
ISFBG, Urban Dictionary
Follback, Urban Dictionary