Reciprocity

Humans tend to return good deeds: use this social psychology law in user interface design to gain users’ trust and motivate engagement with your site or app.

The Reciprocity Principle: Give Before You Take in Web Design
Nielsen norman Group
Applying Behavioral Principles to Personalize Content Using Social Media
UX Matters
Reciprocity (social psychology)
Wikipedia
Robert Cialdini on the importance of reciprocity (video)
WOBI