I always kept quiet about this, but I've felt that way for a long time about anything that comes from Google and especially Facebook. Should I not feel a little icky about using React.js?
People are imperfect, and organizations made of people are inherently imperfect. If you didn't allow yourself to use anything made by anyone who acted immorally, you wouldn't be allowed to use anything.
Should I not use Comcast for my internet if it's the only provider in my area? At a certain point the 'karma' associated with taking a hard-line on a company's ethics is no longer worth the inconvenience.
I don't think there's anything wrong with the team behind it. The reason I might choose not to use a technology owned by Facebook is that my use would be promoting it; the more popular React becomes, the more encouraged people are to contribute code to it, which helps Facebook in a minuscule way. Nonetheless, the popularity of React does contribute to Facebook's success in some way.
Comcast is a different story, as I'm not aware of people helping them for free by using and developing their own tools. I get what you are saying by generally objecting to Comcast and facing the dilemma of paying them when you have no other options. I guess it depends on how much you dislike Comcast and if you are willing to replace them with free public WiFi and Netflix. At least with Facebook, there are plenty of alternatives(as well as ripoffs of React).
Contributing to firefox, openssh, wikipedia and the linux kernel also helps Facebook in various ways. Should we stop doing that?
React is an open source project. Facebook did right by it (unlike other open source projects coming out of FB such as Flow). I find this hardline stance kinda ridiculous when it's one of the rare good things to come out of Facebook. It should be encouraged if anything. But even if not, it is a standalone project and a very good model example of how companies should release stuff as open source, so not contributing to it "because it helps Facebook" is completely missing the point of what open source is.