> In what scenario would pulling in a friend's profile photo create a useful ad?

Exactly in the scenario you just described. You still remember it and you are actively talking about it years after the fact.

Zero people in the process of creating that ad said "we'll suggest people date their siblings, it'll be so memorable"

That is absolutely not a success story when trying to market a Christian dating platform.

Sounds like the viewers were highly unlikely to have clicked through. Cost the advertiser a view but lost the conversion.

Useful ad for Facebook. They made money on it. The advertiser didn't.

wouldn't "useful ad" imply either 1) clicking through and buying the product or service, or else 2) building up a positive brand association to help increase sales later?

remembering an advert correlates but is different to it being valuable.

But it didn't bring clicks to the website nor goodwill toward the company.

No one remembers who ran the ad. Even if we did, it would only be in a negative light due to a weird and off-putting advertising approach.

Many people want to date their own friends? Seeing your friend is on the site would show it's okay to use?

This is a ridiculous argument that just because someone still remembers something means it was a good advertising strategy. This is partly why advertising sucks. The correct metric in this case would be did the user actually go on the date with the said person or at least initiated the conversation. In this person's case, very likely not. So the strategy is dumb, ridiculous and laughable but not useful or good in any sense.