Facebook will always show you an ad. First the system tries to select a relevant ad with the highest predicted income ($$$) for FB winning the auction for your screenspace (the placement). If FB can't find a perfectly relevant ad the algorithm will choose a less relevant ad, But it will always show you an ad.
The thing is FB does not select a relevant ad from all available ads on FB, but only from a subset of ads matching a certain targeting setting that in turn matches your profile or interest. And this targeting setting is set by the uploader of the ad. And in some cases advertisers select a targeting that does not make any sense at all. (They target you by error.)
Does the placement algorithm like to show you a non relevant ad? Most likely not because on average it yields less money for FB. It could be that FB does not have much information on your interests or your privacy settings disallow using this information and the algorithm does not know what is relevant to you. And/Or the advertiser had a very high bid for the placement and the placement algorithm decided a less relevant ad with a high bid yields more money than a more relevant ad with a low bid.
(This is often the case if the advertiser decided to pay per ad-view and not per ad-click. Pay-per-click ads only yield money to FB if they are relevant, because users don't click on non relevant ads. Per-per-view ads always yield money for FB.)
But in the end it's not facebook who is to blame. It's the advertiser setting up stupid targeting and placement bids. (I once burned through 10000€ in one minute because of an erroneous targeting setting while showing the right advertisement to the wrong people...)
The thing is FB does not select a relevant ad from all available ads on FB, but only from a subset of ads matching a certain targeting setting that in turn matches your profile or interest. And this targeting setting is set by the uploader of the ad. And in some cases advertisers select a targeting that does not make any sense at all. (They target you by error.)
Does the placement algorithm like to show you a non relevant ad? Most likely not because on average it yields less money for FB. It could be that FB does not have much information on your interests or your privacy settings disallow using this information and the algorithm does not know what is relevant to you. And/Or the advertiser had a very high bid for the placement and the placement algorithm decided a less relevant ad with a high bid yields more money than a more relevant ad with a low bid.
(This is often the case if the advertiser decided to pay per ad-view and not per ad-click. Pay-per-click ads only yield money to FB if they are relevant, because users don't click on non relevant ads. Per-per-view ads always yield money for FB.)
But in the end it's not facebook who is to blame. It's the advertiser setting up stupid targeting and placement bids. (I once burned through 10000€ in one minute because of an erroneous targeting setting while showing the right advertisement to the wrong people...)