On Instagram, the term #SFS means "shout-out for shout-out." An Instagram user may use the term as a promise to shout-out, or promote, another Instagram user in exchange for a mutual shout-out. In this way, both users gain exposure and, hopefully, gain more followers and likes on their Instagram accounts.
Shout-outs tend to be the most effective when shared between users who post similar content, such as two users who post daily fashion photos. Similar hashtags include #Like4Like and #Follow4Follow. Both are used to increase likes and followers in exchange for doing the same for other users. An Instagram photo with a higher number of likes has a higher likelihood of being a popular Instagram post and being featured on the main page.