The acronym LB stands for "like back," which is a way to get other users to like one of your posts if you like one of theirs. The more likes you get on a photo, the more popular your account looks. Many Instagram users comment "LB" on a celebrity pages instead of their own friend's pages because a celebrity has millions of followers, so their comment have the potential to be seen by more people. Kylie Jenner's Instagram pages are filled with people writing "LB", which she has asked from them to stop doing, but have not.