U.S. users are leaving Facebook by the millions, Edison Research says

Facebook says their North American monthly active users have increased 1% since 2017. Edison Research says there are 6% fewer US users since 2017.

I'm curious to understand the reason for this discrepancy. Edison says,

We're saying, "Do you currently use Facebook?" Facebook is probably measuring it on, "Do you ever open the app, or do you ever use it on any level?"

Here's how Facebook defines monthly active users:

We define a monthly active user as a registered Facebook user who logged in and visited Facebook through our website or a mobile device, or used our Messenger app (and is also a registered Facebook user), in the last 30 days as of the date of measurement.

So is Edison's explanation reasonable? Maybe people only think they "use" Facebook when they scroll through the feed - and people are still using it for other purposes.

using facebook has become a sort of taboo in certain cycles, so people may be misreporting it intentionally

also, a significant drop in users should be detectable by every website which relies significantly on facebook traffic