I get that's closer, but it's still not that useful, because it doesn't actually capture how people use the software and whether that's changing.

People communicate with me via FB messenger, and I respond to them. Otherwise I don't use the app. Am I an active user?

Once a day I open it, realise it's shit and vacuous, and importantly my SIL (the only person I still use it to keep up with) hasn't made any useful updates, so I close it 20 seconds later (this one is me, fwiw). Am I an an active user?

I don't post anything, I just scroll through the same 200 friends I'd had for 10 years, with no movement or differing usage. Am I an active user?

I don't use facebook as a social tool, I have no friends etc, but facebook groups are used in my area to buy and sell things. The second people move to another site I'd happily move. Am I an active user?

    Am I an active user?

Yes

    Am I an active user?

Yes

    Am I an active user?

Yes

    Am I an active user?

Yes

That is a very nicely formatted response.

It--- at least to me--- proves my point that active user is not a useful metric, as all of those people have very different values and drive growth (or don't) in very different ways, and so have no use being clumped together.

We didn't even get to talk about differing user demographics or time, data by country, trends inside countries etc.

You could have made the same criticism about a magazine in 1965. They tout to advertisers how many “readers” they have and how readership is growing, and your criticism is “all I do is open it and flip through the pages; I don’t _read_.”

Your value is not in reading the articles but in viewing the ads. For FB display advertising, you opened the app and viewed ads. Eyeballs captured! Value unlocked! Sure they’d rather have you spend 5 hours on the site, but even your slight attention is valuable.

That's like saying 'pounds' is not a useful metric because it does not tell you 'pounds of what'.

What weighs more, a pound of lead or a pound of feathers?

Pretty sure OP is a passive user, and is extremely susceptible to better tools for OP's needs coming along, eg: the current crop of buy/sell/trade apps/PWAs out there.

There's no such thing as a passive user to facebook through. There are accounts with no activity at all and are simply dead, and accounts with activity. That's it. An active user is simply an account that it still used from time to time. The point is the metric tells you nothing about how it is used.