Getting users to use a new feature by showing others who used the feature isn’t a new concept for interfaces. It’s even more powerful when the other users are friends… because it gives an air of legitimacy to the feature. The technique is probably as old as public markets.
If your friend buys somewhere and tells you about it, well hey, why shouldn’t you do it too? They’ve just taken a lot of the upfront work out of the process for you. LinkedIn does an especially slick job of this with a feature that scans your email contacts and suggests connections.