React, Flux, GraphQL, Hack, HHVM...? All of this and more!
UPDATE: Facebook has released an FAQ on React licensing and it's also adopted by Microsoft and Apple and Apache Foundation has now taken a stance against Facebook BSD+Patents license.
If you are using or considering using React in a project you might want to consult a lawyer. Because of the patent clause you are not allowed to do anything that constitutes as competing with Facebook. If you do take legal actions or in other ways challenge Facebook, your license to use React is immediately revoked.
Your license is also revoked if you have any legal disputes if you have legal disputes with any other company using React. This is the reason why both Google and Microsoft employees are not allowed to use React.js in their work - according to Rob Eisenberg, creator of the Aurelia framework and a former member of the Angular 2 development team.
While this may be a theoretical impact for most implementation projects, it's certainly worth remembering and can limit some other projects like WordPress Calypso which have built a deep coupling to the library. Automattic, the company behind WordPress, is no stranger to petty litigation, but with Facebook they would likely be on the receiving end.
Many Open Source products like Drupal and WordPress are adopting Facebook technologies. It might be tricky when companies like Automattic with Open Source based products like WordPress.com maybe unable to leverage their userbase in the future in ways that could be considered as competition by Facebook.
This is a distant possibility now as Facebook is at the height of it's power, but like Yahoo! Facebook will fall back from grace some day. At that point the company might be interested in taking use of the power granted by the license in a competitive sense. So if you're looking to create a Facebook killer with React, you might want to call up your lawyer.
Remember that patent laws are enormously complex, especially in IP related matters they are often hard to interpret. As the feud between Google and Oracle over use of Java in Android shows, they can take years to handle. This means that you'll potentially need very deep pockets, even if you end up winning the case.
Here is the whole clause any developer using React.js in their projects agree to:
Additional Grant of Patent Rights Version 2
"Software" means the React software distributed by Facebook, Inc.
Facebook, Inc. ("Facebook") hereby grants to each recipient of the Software
("you") a perpetual, worldwide, royalty-free, non-exclusive, irrevocable
(subject to the termination provision below) license under any Necessary
Claims, to make, have made, use, sell, offer to sell, import, and otherwise
transfer the Software. For avoidance of doubt, no license is granted under
Facebook's rights in any patent claims that are infringed by (i) modifications
to the Software made by you or any third party or (ii) the Software in
combination with any software or other technology.
The license granted hereunder will terminate, automatically and without notice,
if you (or any of your subsidiaries, corporate affiliates or agents) initiate
directly or indirectly, or take a direct financial interest in, any Patent
Assertion: (i) against Facebook or any of its subsidiaries or corporate
affiliates, (ii) against any party if such Patent Assertion arises in whole or
in part from any software, technology, product or service of Facebook or any of
its subsidiaries or corporate affiliates, or (iii) against any party relating
to the Software. Notwithstanding the foregoing, if Facebook or any of its
subsidiaries or corporate affiliates files a lawsuit alleging patent
infringement against you in the first instance, and you respond by filing a
patent infringement counterclaim in that lawsuit against that party that is
unrelated to the Software, the license granted hereunder will not terminate
under section (i) of this paragraph due to such counterclaim.
A "Necessary Claim" is a claim of a patent owned by Facebook that is
necessarily infringed by the Software standing alone.
A "Patent Assertion" is any lawsuit or other action alleging direct, indirect,
or contributory infringement or inducement to infringe any patent, including a
cross-claim or counterclaim.
-- Additional Grant of Patent Rights Version 2 for React