The likes are recycled. Bob likes an article on a website. Two years later that website advertises with Facebook. Facebook re-use Bob's like, and post the ad.

Some people 'like' a lot of things and might not remember what they've liked; or they may discover something about a previously liked company but not know how to unlike things or etc etc.

Facebook say you can turn off this behaviour. (https://www.facebook.com/settings?tab=ads&view)

I'm not sure I'd use the word "malicious", but the combinations of obscure settings (how to find the list of liked things; how to unlike things; how to opt out of social ads; etc) is sub-optimal and gives the appearance of sleazy marketing.