Almost every social site and app has equivalents of these: Pinterest says "repin" instead of "reshare". Facebook lets you "friend", but also "subscribe".
The basics: Like, Comment, Reshare, and Create. These are roughly in order of occurrence The dropoff can be up to an order of magnitude with each step: The average Facebook status has far more likes than comments.
People are passive, and creating content is hard. The more creativity it requires from the user, the less likely they are to do it. Write a status? Easy. Snap a photo and apply a filter? Medium. Write an eloquent blog post? Hard.
Great user retention strategies rely on new, useful reasons to notify people. Circa 2007, you used to get an email when someone messaged you on Facebook. In 2012, Instagram notifies you when a Facebook friend of yours has joined. Quora sends you a digest of answers to questions that are relevant to you.
The holy grail: The elusive invite. That glorious moment that pushes the viral coefficient towards 1. Everything has been tried: Gmail brought us the exclusivity of being one of the selected few. Some sites plainly invite you to "invite 5 more friends". Rare is the site like Eventbrite that wouldn't work unless you shared with others. That's the kind of product you want to build.