What Meta says
In Meta's own words: “Website custom audiences are an option for advertisers looking to reach people who have already shown interest in their business.” (Meta help center)
How it is built
You create it in Meta's Audiences tool from your pixel as the source, then choose an event or URL rule: all visitors, or "People who visited specific web pages," which accepts URL matching so you can target a path or a "URL contains" pattern (handy for splitting one page by utm_campaign). You set a retention window in days (up to 180, or up to two years for purchase events) and can layer up to five include or exclude rule sections. One catch: it does not backfill, it only populates from the moment you create it, so set it up before the spend starts.
Why it segments by page, not campaign
The audience is defined by the URL and the pixel events, never by the campaign or channel that referred the visit. That information is invisible to it. So two ad campaigns pointing at the same page produce one indistinguishable pool, and the only way to split retargeting by intent is to split it by URL: a dedicated page per intent, or a UTM parameter your rule can read. Seed a lookalike from a converter audience, not from all visitors. The full playbook is in why your retargeting is built from the page, not the intent.
A worked example
Say 4,000 people hit your /pricing page in the last 30 days and 600 of them bought. A website custom audience with the rule URL contains "pricing", 30-day retention, and an exclusion for the Purchase event over the same 30 days leaves about 3,400 people to retarget, and fewer in practice, since only visitors who use Meta technologies and have not opted out of ad personalization are counted.
Common questions
What is a website custom audience?
It is a Meta retargeting audience built from your pixel data, made up of people who visited your site or specific pages. You define it with URL rules and pixel events, set a retention window (up to 180 days, or two years for purchase events), and use it to retarget or to seed a lookalike.
Can a website custom audience segment visitors by ad campaign?
No. It is keyed to the page URL and pixel events, not the campaign that referred the visit, which is invisible to it. To segment retargeting by intent you have to send each campaign to a different URL, then build one audience per URL.
How do I create a custom audience from my Meta pixel?
In Audiences, choose Create audience, then Custom audience, then Website. Pick your pixel as the source, choose the events or URL rules, set the retention in days, name it and create it. It starts populating from that moment and does not backfill.
How long can a website custom audience keep people?
Up to 180 days for visitor, page and most event rules, and up to two years for purchase events. People drop out automatically once their last qualifying visit falls outside the window.
Why is my website custom audience smaller than my site traffic?
It counts unique people, not pixel fires or sessions, and only visitors who are Meta technologies users; accounts that have opted out of online behavioral advertising are excluded. Meta suggests waiting until the audience has several hundred people before using it.