Prevent products I’ve searched for appearing as Facebook ads on other devices

Question

I’ve recently noticed some things that I have searched for on Google on my mobile device have started appearing as adverts in my partner’s Facebook feed on her mobile device.

I don’t have a Facebook account, I use Firefox with tracking disabled and we always use different devices, but we are on the same WiFi network.

It’s an issue because I am researching things for my partner’s birthday. It’s also creepy from a privacy point of view but that’s by the by.

I’m assuming the similar IP address is the issue but this is the first time I’ve noticed it occur when using separate devices. Is there a way that I can stop this aside from using a VPN?

Answer 1

I expected that this only happens via cookies (and other locally stored state) combined with login accounts since IP addresses are commonly shared, including between parents and children, and your ISP can reallocate them, but see the Update below.

You can protect from the IP address possibility by using your phone's cellular connection (turn off WiFi), or using a cafe or library WiFi (if they run WiFi even when closed during a pandemic), or a VPN (if your partner isn't also using it, as would happen if your router handles VPN for the entire LAN), or an anonymizer service.

Facebook reportedly even tracks people who don't have accounts, so start by clearing all Facebook cookies, clearing the browser cache, and restarting the browser. There's also a Firefox extension Facebook Container that makes it harder for Facebook to use third-party cookies to track your visits to other websites.

Perhaps your browser is logged into one of your partner's accounts or vice versa. The easiest workaround is to use Firefox private browsing mode (or Chrome/Chromium incognito mode). That begins with no cookies and deletes them afterwards.

Be careful about logging into an account while doing this, say Amazon, since that account can track your interests. If you need to log in, consider creating a fresh account or making sure that account isn't logged in on your partner's computer or linked to other accounts.

You can use duckduckgo.com for web searches that never track you. That step is good for web searches but the commerce sites you reach might still track you via cookies, so definitely use private browsing mode.

A way to test these measures is to search for some unusual and different products and services like audio mixers, robot arms, pickup trucks, skis, house painting, estate lawyer, or car repair. See if that "interest" crosses over to your partner's computer.

Update: Twitter's privacy policy says they share user IP addresses and mobile device identifiers with the Google and Facebook advertising platforms! Even if you never use Twitter, this confirms your hypothesis that ad networks associate commercial interests with IP address.

After product searches on Amazon, I've seen ads on other websites for those specific products, not just the categories. Maybe that happens via Amazon cookies or via IP address.