The app - called tbh, meaning "to be honest" - is just nine weeks old, but had already been downloaded 5 million times.
The app's creators said it will remain a standalone program but will now have more resources thanks to Facebook.
"We were compelled by the ways they could help us realise tbh’s vision and bring it to more people," tbh said.
According to start-up news site TechCrunch, the deal was for "less than $100m", and tbh's four person team would become Facebook employees.
'Emotional needs'
