Social Media: What Motivates People to Use Them?

I signed up for Jelly right when its beta opened but almost immediately deleted my account and the app -- but that's another story.
Here in TechCrunch is a prescient piece about what motivates people to embrace social media and social services. Nearly any social media/service requires people to invest at least time and attention; why do some socials stick, and some fade away?
You’d Be Surprised By What Really Motivates Users
Posted Feb 7, 2014 by Nir Eyal (@nireyal)
Editor’s note: This article is adapted from Hooked: A Guide to Building Habit-Forming Products, a new book by Nir Eyal and Ryan Hoover.
Earlier this month, Twitter co-founder Biz Stone unveiled his mysterious startup Jelly. The question-and-answer app was met with a mix of criticism and head scratching. Tech-watchers asked if the world really needed another Q&A service. Skeptics questioned how it would compete with existing solutions and pointed to the rocky history of previous products like Mahalo Answers, Formspring, and Aardvark.
Via http://techcrunch.com/2014/02/07/youd-be-surprised-what-really-motivates-users/
In an interview, Biz articulated his goal to, “make the world a more empathetic place.” Sounds great but one wonders if Biz is being overly optimistic. Aren’t we all busy enough? Control for our attention is in a constant tug-of-war as we struggle to keep-up with all the demands for our time. Can Jelly realistically help people help one another? For that matter, how does any technology stand a chance of motivating users to do things outside their normal routines?
We hope a few insights gleaned from user psychology may help the Jelly makers improve their jam and provide some tips for anyone building an online community.
See the article for full treatment of the motivators and examples thereof -- but the motivators, in summary are,
- Reward system -- recognition from peers or the community may be a more effective incentive than money or prizes
- Frequent engagement -- services that people use daily, and that deliver incentives (rewards) frequently, succeed
- (Related to #1,) the community with which people would enage in the social media/service, is a "community of people whose opinions we care about"