My Facebook and every other site with adtech tends to spend most of its time filled with ads for the last major purchase that I researched online and then bought something for, often advertising the exact thing that I just got one of and won't need to replace for years.

A.K.A the Amazon Suggestion Effect ("You just bought a fridge, here are 12 more fridges!"). I thought Amazon was keeping their suggestions dumb on purpose, so as not to appear too creepy, apparently I was overestimating how smart adtech is in general.

I always assumed they showed those ads in case the order was cancelled or returned.

Never attribute to intelligence that which is adequately explained by stupidity.

It took about six months for the online mattress ads to settle down after I quickly (within a day or two) researched and purchased one. I recently purchased an expensive pair of headphones and I suspect to see those for the next several months.

Often its not even smart enough to pick up on things you're really going to buy. A few months ago I did some research on designs for folding wooden tablet stand to base my own build on, and one of the images I clicked on from Google Images shows up to this day as one of the prominent ads on every Wikia fansite I visit with that particular computer. I didn't even visit the store it's sold on, and they're still clinging for dear life to this one scrap of information on what I might buy.

Do you use adblockers or other privacy tools? It'd possible that marketing companies simply haven't recorded information beyond your Facebook friends.

Anecdotally disabling blockers drastically improves relevance... though I still block them because relevant ads are as bad as untargetted ones.

they know, they just dont care. ads on fb are ridiculously cheap. They have soooo many impressions relative to the number of advertisers. So your furniture company says 'target everyone over 21 cause why the hell not, it costs barely more than doing an intelligent campaign'. Also, their click through rates are terrible, so you end up needing to spam as many impressions as possible.

Funny. Facebook is scarily good at targeting me, to the point of it sometimes being almost unnerving. All this despite the fact that I'm on facebook maybe a couple of times a week and post stuff maybe a couple of times a year.

Facebook is probably one of the only sites where I've seen an ad and gone "wow, I was actually looking for one of those"

I sometimes get that, if I was looking something up on a shopping site earlier in the day, from that same computer. It's almost always the thing that I was looking up for someone else, or the thing that I bought the week before. There's rarely something that seems particularly thoughtfully targeted.

Then again, I have a policy of not "liking" any kind of brand, group, or anything, don't post links to outside sites, and I randomly mark ads as offensive.

The unique thing ad tech does over traditional is provide info on if you clicked the ad, visited the site, etc. marketing to specific demographic is nothing new.