I was contacted by a facebook recruiter a few years ago. I started the process and also started thinking about if I would like working there. I realized that I disagree strongly with the product, its effects on society, culture and politics. But more importantly I realized I hadn't used the product in like 3 years. I just said no thanks before even finishing the "take home" exercise.
At the time I was not a college grad, I was a team lead at a startup. This was before all of the recent news (2016?). It turns out I was right to make that decision.
I think there is strong incentive for engineers who commit to working on projects which they would actually use themselves. I am not overly ethical as a person (although I do have opinions I try to stick to), but I am very happy to have the opportunity to work at companies with products I actually use.