It’s getting pretty close.
“If a company has a market share of greater than 75 percent, they will probably be considered a monopoly.”
https://business-law.freeadvice.com/business-law/trade_regulation/monopoly_power.htm
“In January 2018, Facebook had a social network market share of 70.88 percent. Furthermore, it had the greatest market share of social networks that month.”
https://www.statista.com/statistics/280301/market-share-held-by-facebook-in-the-united-kingdom-uk/
“If a company has a market share of greater than 75 percent, they will probably be considered a monopoly.”
Are 75 percent of the people with internet access on Facebook?
“In January 2018, Facebook had a social network market share of 70.88 percent.”
How would you define a ‘social network?”