Did either of you by any chance send links?

Even if message content is encrypted end-to-end, I'm curious if those previews it loads are tracked.

Just curiosity. Can't really say they are, but it might explain something.

Good point. No links sent.

As far as I can tell and given how unstable link previews are on WhatsApp, I think the request is sent from the device itself. But don’t know for sure. (easy to check though by sending a link to your own server and checking IP logs)

Compare to the preview experience on Telegram for example, which I’m sure is using its own servers to send the request given how quickly the preview usually appears (i.e. it is cached).

Could be much simpler: the service app uses Facebook marketing tools at some point (in the SDK or server-side) for ads and retargeting.

So when you opened it you showed some intent that was deemed interesting enough.

The explanation makes sense on a technical level.

But if the intent came from within the service’s app (i.e. I’m already a customer), why offer me an ad for the same one?

Depends on how the marketing people defined their funnels. Maybe they thought that people who showed interest but then left the app, are more likely to come back if they saw it later elsewhere on the web. But at this point it is just speculation

It is not necessary to read your messages to target you with an ad. All that Facebook needs to know is that you and your partner were talking. It's very likely that your friend visited the bike rental service website, Facebook page or chatted with them. The metadata of your chat, which is not encrypted, could reveal this information, such as when you and your friend were speaking and how long the conversation lasted.

This is technically plausible.

I’m not an ad expert but how often do ad networks work associatively? I.e recommend something to person A because friend B did something?

At some point Facebook was recommending me friend-suggestions just because we were using the same wifi.

So I'm guessing they're digging deep into that relationship tree.

They don’t need to read your messages. Your behaviours outside of the messages are enough and they fact that you messaged each other.

Some simple explanations from there on out:

- your partner searched for the bike service or just any bike services and they happen to be the best fit for ads. More so, if you mentioned it to any friends, they could have searched for it and Facebook can associate relationships.

- neither of you may have had an original thought when the discussion of using the service came up. You maybe heard the idea in passing and/or were in an area where others were searching for it.

Facebook doesn’t need to know what you talk about, as much as they need to know who you talk to and building relationship models from there. Everything after that is simply extrapolating that you have interests similar to the people you interact with regularly OR to the people who go to the same places as yoy

The Social Dilemma and The Great Hack documentaries on Netflix cover this in greater detail but basically they don't need to read your messages, when the can predict your behaviour so accurately. IMHO this is actually worse than reading your messages.

Sometimes I get ads on YouTube for a restaurant that I was just thinking about. But 99.9% of the time, the ad has nothing to do with anything.

I think it's just a coincidence.