So far, edX's offerings are heavy in technical fields — for example, circuits and electronics (MIT), introduction to computer science (Harvard) and foundations of computer graphics (Berkeley).
EdX officials say they plan to roll out new courses soon, including some in the social sciences and humanities.
"I actually think we're moving quite quickly," said Michael D. Smith, dean of the faculty of arts and sciences at Harvard and a member of the edX governing board. "We've got a pipeline of courses coming out now. You'll continue to see us accelerate."
EdX is nonprofit, while Coursera is for-profit. That may explain, in part, why Coursera is growing at a faster pace. Both platforms are considering various ways to generate revenue without charging tuition.
Analysts say that edX is seeking to carve out a high-quality niche in the market.
"The interesting question is, how good is the thing that's free?" said Kevin Carey, an education analyst at the New America Foundation. "It doesn't surprise me that they're being deliberate about it. If they're putting their brand names behind this effort in a very public way, you absolutely want to mitigate against the risk of people saying, 'This is no good.' "
Jeff Selingo, editor at large for the Chronicle of Higher Education, said MOOCs could evolve into a recruiting tool.
"Remember, all these universities scour the world for the most talented students," Selingo said. "They're always trying to find that diamond in the rough."
Eventually, he said, universities may weigh whether a prospective student has passed a MOOC: "Why shouldn't that be considered as part of your portfolio for admission?"