Also standalone: https://www.meta.ai/ and https://about.fb.com/news/2025/04/introducing-meta-ai-app-ne...

I use Messenger but not Facebook. There's a big "Ask Meta AI or Search" bar at the top of the app's main page.

Whatsapp you can tag @meta ai in any chat, have a separate chat with meta ai, forward messages to the meta ai chat for fact checks, etc.