Like for like

Nearly every celebrities instagram receives comments like these above. Users comment on photos (first for first, like back, follow for follow) stating that if other users like or comment on their pictures then they will return the favour on their account. This appears to be a form of reciprocity, as by doing something for someone else the other person feels obliged to give something back. 

Regan (1971) studied the concept of reciprocity by measuring the extent to which participants complied with a request. 77 participants rated the quality of paintings with a confederate, whom behaved normally and politely (pleasant condition) or rudely and unpleasantly (unpleasant condition). Throughout the experiment, the confederate left the room and either returned with a soft drink (favour condition) or with no soft drink (no favour condition) for the participant. Alternatively, participants received a favour from someone else, who was not the person who would later make a request (irrelevant favour condition). After the task was complete the confederate asked the participant if they would buy a raffle ticket, with the amount of tickets bought reflecting the level of compliance.

As shown in table 1, participants in the favour condition, where they received a soft drink from the confederate, bought more tickets on average than those who did not receive a soft drink. This study illustrates that people tend to repay favours after receiving a favour themselves, regardless of whether they liked the person, as no significant effect was found between the liking conditions. 

In the instagram example, users feel obliged to like or comment back on other users' photos, which can be explained by the reciprocity principle. Regan's (1971) study can also explain why people repay favours to people they may not know or like, as the relationship between favours and compliance does not seem to be affected much by liking of the person. People simply feel obliged to repay another person a favour.

Reference
Regan. D. T. (1971). Effects of a favour and liking on compliance. Journal of Experimental Social Psychology, 7(6), 627-639.