I think the OP refers to Facebook waiting really long to offer a mobile app. Many others jumped into the gap (I personally used an app called "Friendly for Facebook").
They were indeed really slow to recognise mobile as a worthwhile platform. So long that it would have jeopardised their position if it hadn't already been pretty much invincible.
They released a dedicated iOS app on the same month the App Store launched (July 2008), about a year after the first iPhone was launched. They had m.facebook.com (dedicated mobile site) in time for the iPhone launch (Remember when Steve Jobs said web apps were the future? I remember!)
Messenger as a standalone app was released in 2011. They bought Instagram pretty much within two years of birth.
The UX of a lot of the mobile products might not have been good - I don’t remember - but all things considered I would say they moved fast on mobile, at both the engineering and strategic levels, and it paid dividends.
They were indeed really slow to recognise mobile as a worthwhile platform. So long that it would have jeopardised their position if it hadn't already been pretty much invincible.