Shopkick, a mobile retail startup based in Palo Alto, Calif., just announced that it has raised $15 million in a second round of funding. The round was led by Greylock Partners, and LinkedIn founder Reid Hoffman is taking his first full board seat as a Greylock partner.

In December, Shopkick released an iPhone and Android application called CauseWorld, which allows users to earn money for charity by “checking in” to stores and restaurants. But that was just an experiment to test out the concept behind its main Shopkick app, which the company plans to launch this summer. Cofounder and chief executive Cyriac Roeding described the coming app as basically a “selfish” version of CauseWorld.

CauseWorld was downloaded 550,000 times in five months. Roeding said the apps’ users taught Shopkick a lot about what check-ins mean in a retail setting. Shopkick wants to make a “big push from the check-in to the check-out” — in other words, it doesn’t just want to enable interactions in the store, but also after you’ve purchased a product and taken it home.

Shopkick has been around for almost exactly a year, so Roeding described the funding as “kind of a birthday gift — the nicest one we could imagine.” The company previously raised $5 million from Kleiner Perkins Caufield & Byers‘ iFund and from Hoffman, who invested as an individual. Kleiner, Citi Growth Ventures & Innovation Group, and Ron Conway’s SV Angel also participated in the round.

Hoffman, who joined Greylock as a partner in November, said the funding will hopefully give Shopkick the money it needs not just to launch its app, but also to hit cash-flow positive (where more money is going into the company than going out).