I always got the impression that downdetector worked by logging the number of times they get a hit for a particular service and using that as a heuristic to determine if something is down. If so, that's brilliant.
When Facebook's properties all went down in October, people were saying that AT&T and other cell phone carriers were also down - because they couldn't connect to FB/Insta/etc. There were even some media reports that cited Downdetector, seeming without understanding that they are basically crowdsourced and sometimes the crowd is wrong.
Some 3600 people have hit that button in the last ~15 minutes.