What are Bloom filters?
Jamie Talbot

I love how the filters fit in this particular problem very nicely. However, I would have loved it if you could say what amount of bits you use per user. Maybe something like 1kb (8000 bits)? And maybe you reset it all to zero after a while, depending on how frequently the user visits Medium? E.g if he comes once a day clear it after a week, if he comes once each month clear it after a month?