I'm not convinced that is sufficient. Facebook at 80% isn't really any different than facebook at 99, or whatever. Google has a viable competitor in Bing, but google still dominates search to a massive degree.
As long as it's there, then if there's every a major ball drop on Facebook's part that makes a lot of people want to switch to something else they'll have somewhere to go. When people moved to Facebook from MySpace it was driven by a lot of issues with MySpace that pushed people away.
Something like this past year's election with all of the controversy over how the trending news was handled could have promoted a movement of "Enough, I'm done. Going to X." Enough "me too's" and you start a trend.
A lot of that happened with Instagram. People wanted a place they could go free of political rants and Instagram largely provided that...and Facebook owns them.
You've seen a lot of "quitting Facebook" trends this year and I believe that it's largely a result of lack of a viable alternative.
They need something on the other side of the "or" in "Uses Facebook or ..."
They do their best to buy anybody that sneaks in there.