Facebook has limits in place to prevent abuse of their features and to protect people from spam and harassment. For example, if someone is sending out a bunch of messages to people they aren’t friends with, they may be warned or temporarily blocked from sending messages.
Limits are based on different factors, like speed and quantity, but Facebook can’t provide additional details on the rate limits that are enforced.
If you’re blocked from using a feature on Facebook, you can learn more about feature blocks.
Source: https://www.facebook.com/help/177066345680802?helpref=uf_permalink