@Gargron I say the following (whenever I try to do this exercise it only passes if my mother-in-law can repeat it to a friend):

It's an interlinked network of communities owned and operated by the communities they serve, with millions of members around the world.

I use as little techspeak as possible

I do not highlight the negatives of other platforms, 99% of the people being spoken to don't know or don't care.

I do not highlight similarities to email as I don't want to confuse the use case