Follow back
Social norm: if someone follows you, follow back. Abbreviated "FB" or "F4F" (follow for follow). Standard reciprocity expectation on Twitter/Instagram pre-2015; less mandatory 2020+.
Mass-follow-back bots (2010s) wrecked usage: script follows 1000 random, waits a few days, unfollows those who didn't follow back. Mass-follow tracking tools (Crowdfire, etc) made this industrial. Instagram banned several apps. Modern strategy: no follow-back norm — selective follow is social signal of curation. Power users on Twitter often follow 200-500, have 100K+ followers — asymmetric ratio is new status signal. "Follow-back-Friday" is legacy hashtag.