(My favourite app is CityMapper - place your bets now on whether they be bought by Facebook or Google)
Metafilter is a forum populated by interesting, intelligent people. anyone can access it, but you pay a one-off fee to be able to post or comment. There are usually about 30 posts a day, and each post has comments on it, usually greatly adding to the discussion. It's moderated, so any posts that seem to be spammy, or comments that don't really add to the discussion, are removed.
This is an example of a good post - speculation on today's Apple World Wide Developer Conference.
(I've made a few posts - this is a post on Grayson Perry's Reith Lectures from last year)
But... Now Metafilter is losing money, and may ultimately be under threat.
This very frank post on Medium by founder Matt Haughey details the problems, including falling ad revenue, falling traffic due to Google algorithm changes, how they saved costs by moving the hosting to the cloud, and how they've had to cut staff.
However the article ends positively - once the problems were known, users started to donate money of their own volition, to help the site. You can donate here - either a monthly payment, or a one-off amount.
So far nearly 3,000 of us have donated - the site has been very open in listing numbers - and I think there is a lesson for other online businesses. Advertising income is declining, and becoming dominated by big players like Google and Facebook. If people are asked to donate, some (hopefully enough) of them will. But let them know about it, rather than just announcing a closure.
Here's the link to the donation page.