I tested FB messenger bots from different companies...They are very intrusive. They need to limit the messages somehow and also make a clear difference between messages from bots and friends, like gmail does with promotional messages.
Definitely an incentive problem there. The bot must do "something" to justify its existence and attract attention to the fact it is useful, while simultaneously not being so annoying that it is, well, annoying. I'm not sure even a human could hit that window consistently, especially as it's probably a window of negative width (i.e., outright impossible) for a lot of users.
