w/r/t selection bias, "As of the third quarter of 2016, Facebook had 1.79 billion monthly active users."
https://www.statista.com/statistics/264810/number-of-monthly...
Their sample pool covers better than 25% of the human population.
Their sample pool covers better than 25% of the human population.
If nothing else, it's worth considering the demographics: within the USA, women use Facebook at a higher rate than men; people with some college education use Facebook at a higher rate than either college graduates or adults who've never been to college; Facebook use rate is roughly negatively correlated with income level; use of Facebook is negatively correlated with age (http://www.pewinternet.org/2016/11/11/social-media-update-20...). So the leftover US population skews male and slightly higher-income than the Facebook population. And globally, Facebook use is correlated with national wealth (minus some outliers). So there's clearly a sampling bias, and that's worth understanding.
That said, whether the results would be any different is hard (impossible?) to say without actually trying a different sampling methodology, and I suspect the results from this study are valid, sampling bias notwithstanding.