Facebook personal message limit reached.

Hi everyone.
i don't have any knowledge or experience regarding the use of facebook. I recently started to message unknown with similar interests to let them know about the services I provide. However the after sending a few messages I can't send anymore.
Is there any rule I should follow like 10 messages per hour or something?

Please let me know about the method you follow. Thanks!

It depends if these people are already part of your friends list or not. If they are already part of your list then there usually isn't any limit (Of course this depends on if you are simply sending text content or full of links and images)
If they are not part of your friends list, and you continue to send spammy messages, and the recipient reads and ignores it, or reports it, then you will be flagged. Apparently there was a limit of 150 messages per day but I'm not sure if this still is in effect.

If you do not have it from friends, it is normal for that to happen, it happened to me before the same