Because why would you use it over Google? That's a lot of effort to build and launch a likely poorly differentiated service that they'd need to spend a lot of money on to steal users from Google. Why would a user prefer Facebook to Google for search?
Because a huge portion of the world already spends all day on facebook, instagram and whatsapp. All they have to do is integrate web search into their apps.
I'm sure they already have most of the web indexed and search history would give them amazing information about their users.
