[snikolic]

Former VP-Eng at a product-ads ad-tech company here...

You can think of this as a two step process. First, connecting your Facebook UID to structured data about a specific product in a product catalog. Second, connecting your FB UID to multiple devices/browsers without cookies.

I think the second part (cross-device matching) has been explained well by other commenters: tere are multiple techniques involving IPs, hardware footprints, browser footprints, browsing habits, etc.

I want to clarify a few things about how the first part most likely occurred. There's been a lot of emphasis in discussion on the FB "Like" button. It's true that this is a possible way for FB to observe you have visited a specific webpage. However, it's more likely that there was a Facebook "pixel" on a retailer's website (some commenters have been referring to this as "javascript" or "retargeting"). Most e-commerce sellers use these today. They're basically a FB web endpoint that the retailer can pass structured metadata to that lets the retailer communicate to FB that an event has occurred on their website. FB allows retailers to send all kinds of metadata about all sorts of events - page loads, add to carts, checkouts, purchases, in-app events, and custom events. The retailer can also send very detailed info about the content being interacted with on a webpage, down to sub-SKU granularity (e.g. not just a particular shoe, but a specific color/size/variant of that shoe).

Historically, the FB web endpoint would return a 1x1 transparent image so that a retailer could embed it on their website's HTML and a customer's browser is "tricked" into loading the image from a third-party domain. Thus the name "pixel". This is still frequently done, but nowadays the endpoint may just be a REST endpoint and/or may be called via AJAX (or via an SDK within a mobile app).

Facebook also allows retailers to upload their Product Catalogs to Facebook. These are basically a CSV of structured metadata about every product the retailer has for sale. Then, when the retailer sends a pixel event to say SKU 12345 has been interacted with by a user, Facebook can reference that SKU in the retailer's Product Catalog to learn all kinds of info about it.

A really interesting exercise is to install the FB Pixel Helper extension for Chrome (I'm sure there are equivalents for other browsers). It will show you all FB pixels loaded on a given page and what metadata was passed along. Keep an eye on it as you browse the web, especially the next time you browse an e-commerce website. Facebook basically sees everything that happens. They may as well be ingesting everyone's Apache/Nginx logs. :-P

https://chrome.google.com/webstore/detail/facebook-pixel-hel... (Note: I'm not affiliated with this extension in any way)