The following monthly chart of FB shows that price has plunged to near-term support defined by its longer-term uptrend channel median and 23.6% Fibonacci retracement level (166.22).
The reversal from its all-time high made last month has been accompanied by accelerating (to the downside) Momentum (peaked in October 2017) and Rate-of-change (peaked in September 2017) technical indicators which, in fact, reversed well ahead of price.
Neither of these two indicators have broken below their respective zero levels since the early months following FB’s IPO. A drop to, and hold below, zero on this monthly timeframe on these indicators could spell serious trouble for FB in the longer term. In this regard, watch for any drop in price to, and hold below, its next support level around 148.00 (channel bottom plus 40% Fib retracement level).