I was thinking about this topic, probably because my app has (at last count) three “Share” buttons. Whether they will be used at all, or only used to mock the app, I’m not yet sure. The possibility some would want to share what they saw in the app, prompting discussion of the app on social sites, made it clear I should put them in.
Aside: the ALS bucket challenge is interesting, as it wouldn’t have exploded without video cameras on phones and free video sharing social media sites. A few years ago, it would have been a blip on the news.
Are you using viral/sharing features in your apps beyond referral discounts?