Full Answer

Facebook Pixel is a browser-side JavaScript snippet. When a visitor loads your page, their browser executes the script, which fires events back to Meta's servers. Ad blockers intercept this at the browser level — EasyList and uBlock Origin, the most widely used filter lists, explicitly block connect.facebook.net and associated pixel domains. For those users, the Pixel never runs at all.

The practical impact: Meta's optimization algorithms see a distorted version of your conversions. Custom audiences miss blocked users. Lookalike audiences are built from the 60–70% of customers whose browsers allowed the Pixel to fire. ROAS calculations undercount real conversions.

The fix is Facebook Conversions API (CAPI): server-to-server event delivery that bypasses the browser entirely. Running Pixel and CAPI together with deduplication enabled gives Meta the most complete signal — the Pixel captures what it can client-side; CAPI fills the gaps server-side regardless of what the visitor's browser blocks.