(Someone had a post asking a good question about what this means. It got downvoted and deleted, which I think is unfortunate for a community that cares about learning about the world around it, so I'm replying here.)

There are two separate kinds of legal protection at issue here, copyright and patent. The only thing that's close to covering ideas is patent law, not copyright law. (I'm assuming US law here, since that's where Facebook and I both are, but most of this is generally true I think.)

Copyright covers a creative expression, and is automatic. If I write a story, paint a picture, compose a song, or code a program, copyright secures my ability to commercialize that creative work as I see fit. If someone else writes a similar story, paints a similar picture, etc. but does not use my words or notes or lines of code, they aren't infringing my "copy right", because they're not copying my work.

Patents cover inventions, and are very non-automatic and best acquired with lawyers. Their term is also much shorter (about a decade, instead of about a century). US law currently lets you patent an algorithm or an arrangement of computer systems, under the artifice that you're actually patenting the implementation of that algorithm on any physical computers. Patents are issued for the invention, not for a specific implementation, and any implementation that works the same way infringes the patent.

(Trademarks, for reference, cover names used in advertising, i.e., "trade marks". They're somewhat automatic, they only cover names, and they're not very often relevant to F/OSS licenses.)

Traditionally, F/OSS licenses have been primarily concerned with copyright, partly because several of them were written when software copyrights were well-acknowledged and enforced, but when software patents were rare or nonexistent. This leaves you in the unfortunate situation where e.g. Facebook can write some code (automatically gaining a copyright), get a patent on the idea, open-source the code, and then sue you for patent infringement despite your copyright license. To avoid that, Facebook has an explicit patent grant in addition to the copyright license. (Although, as other commenters have pointed out, F/OSS licenses that don't specifically restrict themselves to copyright can potentially be read as a implicit patent grant if you stare at it hard enough.)

At no point in this does Facebook claim ownership of your work. Certainly your idea isn't legally owned by anyone, either you or by Facebook, if you haven't gotten a patent on it. Facebook's patents remain Facebook's patents, with or without this clause.

Because patents cover any implementation of an invention (whether or not the implementor knew about the patent), and because patents are in stupidly dense legalese, it's super common for people to be inadvertently infringing a bunch of patents. The traditional solution for this is that large companies have defensive patent portfolios in a sort of mutually-assured-destruction scenario: if Facebook sues Google over a patent, Google will countersue over all the patents they have. So there's an armistice, at least among big companies. The explicit patent grant weakens that, because now Google can just incorporate parts of React into their React-competitor, and Facebook loses a patent from their defensive portfolio.

So a few clauses like these are normal: you get a patent grant from Facebook, as long as you don't sue Facebook over (different) patent infringement. This maintains the armistice between big companies, and it's also great for the little guy, who wasn't going to sue anyway because they don't have any/many patents (but also doesn't have a defensive portfolio because they don't have any/many patents). These clauses only cover claims of patent infringement, which are the thing that everyone is unintentionally doing en masse, not copyright infringement, which is much harder to do unintentionally, so they're pretty fair.

But the fourth clause is super weird. It means that you can't dismantle any of Facebook's patents by e.g. providing prior art, without losing your patent license. It might even mean you can't lobby for software patent reform. It solely holds up Facebook's patent portfolio and the armistice between big companies. Usually dismantling patents is a good thing for everyone, because it's disarmament, and it's unfortunate this license doesn't extend to people who do that.

(IANAL, TINLA, corrections graciously appreciated.)