It's weird I still see ads for stm forum membership on my Facebook account. The ad copy is like "discount for stm members" and I go to the sales page of stm after clicking there. I bet stm can save a lot of money by setting this straight by exluding paying members from its targeting.
Do you happen to be on iOS 14? Lol
I use to get a lot of STM ads even after I joined. Looking at leads, Facebook is far from perfect in tracking and building custom audiences. I've always excluded leads from the past 30 days, yet I routinely see a duplicate lead 10, 15 or 20 days later.
@matuloo Great. 
And you seem to be one of those who got through the net 
I think this is pretty normal to happen. I'm getting this on my own ads for TheOptimizer as well.
Even though for some specific ads (which I know with 100% certainty) should not be served to TheOptimizer users, since they are excluded there, FB still keeps on serving ads to them.
I would speculate this happens due to the two possible scenarios:
1 - Being or claiming to be user experience focused, FB may serve the ads even to users that have already purchased a service or whatever is behind the ad, with the aim of getting some feedback from them. Any negative experience shared in the comments by someone that has already converted, should penalize the ad by either limiting the reach or increasing the cost per results.
2 - If the audience is pretty narrow and the allocated budget covers up the targeted audience easily, they know they are serving the ads to a relevant audience.
On a side not, it isn't that bad for ads to be served to users that have already converted. I can help on reminding them to use the service and goods they opted for 
@matuloo @platinum thanks for sharing.