Facebook Blocks Ad Blockers, but It Strives to Make Ads More Relevant
nytimes.com
nytimes.com
Is it really that costly and laborious? Just take the hash code of the image and see how many users have already marked that hash code as an ad.
Not trivial, but also not difficult.
Facebook ads are targeted, so not all users will see the same image at the same time, so the largest crowd sourcing benefits would be gone - as opposed to a div id.