In reply to Stephen Edgar.

The license issue is much more serious than Steven L. gives credit. Paul Siemenski notes:

The companies with the greatest concern are those that have large patent portfolios and engage in offensive patent litigation, especially against Facebook.”...“Automattic isn’t in that boat, and has no plans to be, so we’re comfortable using React under its current license.

There are three issues with the insouciance above.

the license conditions might change for a future iteration of React.js. See Google's Chromium for how to boil an open source frog.
Automattic may be acquired by a company who is not on friendly terms with Facebook.
Facebook may go malignant at some point and want to undermine WordPress.com and Automattic as a competitor.

Surprisingly (as he was quite dickish at the beginning), Mark Zuckerberg has shown himself to be a responsible steward of Facebook. Even with all his billions, Zuck is not immortal. It's unclear who will take over Facebook later and what the relationship with Automattic and WordPress will be. Remember Microsoft's primary market for software (when the got started) was Word on the Mac (back in the early nineties). Then Bill Gates decided he wanted to Apple's lunch and jumped into the GEM interface OS business. In case you weren't there, MS almost put Apple out of business but took pity at the last minute (mainly due to the risk of additional anti-trust suits with NO competitor left standing in the OS wars).

In any case, the REACT license is not open source in the sense of GPL. It's an encumbered license and probably shouldn't be at the core of new WordPress functionality when there are real open source alternatives.