The reply without comment:

Hi Marco,

Facebook has limits in place to prevent behavior that others may find annoying or abusive.  These limits restrict the rate at which you can use certain features on the site.  You have received a warning because Facebook determined that you were going too fast when sending messages.

If you have also been blocked from sending messages, please note that these blocks can last anywhere from a few hours to a few days.  Unfortunately, we cannot lift the block for you.   When you are allowed to resume using this feature, please proceed with caution to avoid reaching the limit again.  Please be aware that the threshold at which you are warned is not a specific number, but rather determined by different factors (such as speed, time, and quantity).  For security reasons, we are unable to provide additional information about this system.  We apologize for any inconvenience.

Thanks for your understanding,

Landon
User Operations
Facebook