I'm not that sold on this, I feel as though we hear about this every year that the market has finally topped out and then it doesn't. Its been a very strong earnings season in the US, not sure what FB has to do with representing the whole market all of a sudden. I still remember the FB IPO at $36 being called overvalued because its a company with nothing behind it. Oh the short term thinking does funny things to your brain.
There are people who have been on the sidelines from 2016, maybe even earlier that have lost out on once in a life time gains with the mentality that once it drops they'll come into it which I don't believe at all.
Time in the market > Timing the market