Or it's possible the split up company doesn't have significantly lower market value than the monopoly company. Imagine a 25% non-monopoly discount and a 25% chance of being split up and that would only be a 6% drop in value.
Conglomerates tend to be valued less than the sum-of-the-parts, getting control of conglomerates to spin off their businesses is a common active investing tactic. Breaking FB up is likely to unlock value for shareholders rather than hinder it. At least, short term as far as stock valuations go, a "markets are irrational" stance works in favor of this theory as it (the market) gets to speculate on every sub-business individually. Long term there are advantages to being under a single banner, like subsidizing Oculus growth and R&D through profits from the social medias to capture long term market share (like Amazon can subsidies retail with AWS, Alphabet can subsidies its "Other bets" with Search, etc).
