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.
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.
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.