The market has expected this for years. Look at Facebook's declining P/E ratio.[1] 81 in 2005. 22 now. Successful established companies have P/E ratios in the 10-20 range. Facebook is coming in for a landing as an established company, ending its growth company phase. This is a normal part of the corporate life cycle.
[1] https://www.nasdaq.com/symbol/fb/pe-ratio