>the people I knew who bought in most to G+ were pro photographers.
> meaty in-depth discussions
It's not a stretch to connect the dots there, though. Pros weren't looking for a place to post their latest selfie or a buddy passed out next to the toilet.
I belong to a couple of FB groups and a forum (same subject for all: collectible vehicles). The forum is the oldest (both in terms of founding date and user demographics). It is, without question, THE place to go for technical knowledge. I'll add that the forum format works well for cataloging this type of discussion. Even though it is superior on all counts, it is getting killed by the FB groups. The primary reason, I suppose, is the omnipresence of FB in millennials' lives but ease of posting photos is a very close second. (Add to that the way FB scrapes linked pages and inserts them as images.)
>Facebook (where, of course, many users have almost no income at all
That was among the first things I noticed when I started FB. I thought the elder forum members were cheap. They look like spendthrifts compared to FB users.