IF Facebook has a patent on, say, the virtual dom, then they can sue you if you use Preact, or Vue, or any other library that uses a virtual DOM. And they can just outright sue you; no need to wait until you sue them and then countersue. You are, in fact, more vulnerable if you use Preact or Vue than if you just use React.

...if Facebook has a patent on the virtual dom. In point of fact, they appear not to have any patent on anything underlying React, so none of this matters one bit. The time you've spent reading this comment is already more time than this non-issue deserves.