When you work with facebook messenger please be aware about the few compliances to be followed in order to avoid getting blocked by facebook.
As per facebook OPT-IN policy a user must start the conversation in any of the following ways;
From your page "Message" button
By sharing the messenger shorten link i.e m.me/<yourpage> and when user clicks it opens the messenger app
Integrating Messenger plugin into your website / blog
You have 24 hours to respond to a user through messenger once they wrote you i.e after 24 hours you could not able to contact them until they send you another message.
Here is an overview of facebook standard messaging, message tag, and subscription messaging policies that will apply starting January 15, 2020
Details Extracted from Facebook Platform Polices Document
Standard Messaging - Businesses will have up to 24 hours to respond to a user. Messages sent within the 24 hour window may contain promotional content. We know people expect businesses to respond quickly, and businesses that respond to users in a timely manner achieve better outcomes. We highly encourage businesses to respond to people’s messages as soon as possible. The +1 message that allowed businesses to send 1 additional message after the 24 hour window will no longer be available starting Jan 15, 2020. This change aligns with people’s expectations of faster responses from businesses.
Message Tags - Enable businesses to send important and personally relevant 1:1 updates to users outside the 24 hour Standard messaging window. We are simplifying message tags by reducing the number of tags from 17 to 4 to support certain use cases. Businesses can start using these 4 new messaging tags right away. The new message tags include a Human Agent tag (in Closed Beta) that allow businesses to manually respond to user messages within a 7 day period. Learn more here.
Subscription messaging - Only Pages that are registered with the Facebook News Page Index (NPI) will be allowed to send non-promotional subscription messages. We highly encourage relevant pages to submit their application immediately to register with the News Page Index to allow sufficient time for review and approval before the new policies go into effect.
Sponsored messages - Sponsored messages allow businesses to send promotional content outside the standard messaging window. Sponsored messages are subject to ads integrity controls and the auction dynamics, which work to protect the user experience and ensure the volume of promotional messages is consistent with user expectations.
Here is an overview of the Standard Messaging policy that will be in effect from January 15, 2020.
Businesses will have up to 24 hours to respond to a user. Messages sent within the 24 hour window may contain promotional content. Users have the option to block or mute a conversation with a business at any time.
Here are examples of the user actions that open the 24 hour Standard Messaging window:
User sends a message to the Page
User clicks a call-to-action button like Get Started within a Messenger conversation
User clicks on a Click-to-Messenger ad and then starts a conversation with the Page
User clicks on an m.me link with a ref parameter on an existing thread
User posts or comments on a Page. See Private Replies
User reacts to a message. See Replies and Reactions
Message tags enable sending important and personally relevant updates to users outside the 24-hour Standard messaging window, for a set of approved use cases. For complete details on allowed use cases, see the list of supported Message Tags. Use of tags outside of approved use cases may result in restrictions on your ability to send messages.
On August 29th, 2019 we announced that, effective January 15, 2020, only Pages registered via the News Page Index (NPI) are allowed to send subscription messages.
Subscription messages allow news publishers to send regular news updates to their subscribers in Messenger. This feature is only available for registered news Pages under the Facebook News Page Index (NPI). Registered news Pages may opt in to use the subscription messaging to send messages outside the 24-hour standard messaging window in Page settings. Pages registered with the News Page Index don’t need to apply for subscription permission.
Subscription messaging apply to news content only, and must not be used for promotional content, including but not limited to subscription offers, deals, coupons, discounts or content produced by or that promotes a third party (e.g., branded content, affiliate marketing). To send subscription messages, NPI registered pages need to use the non-promotional subscription message tag. To learn more, see subscription messaging.
Sponsored messages allow businesses to reengage with people who have an open conversation with their Page in Messenger. Sponsored messages can be sent outside the 24 hour standard messaging window and can include both promotional and non-promotional content. Sponsored messages are subject to the integrity controls and user caps, which work to protect the user experience and ensure the volume of promotional messages is consistent with user expectations. Sponsored messages are annotated in the conversation with the word 'Sponsored' above the message.
For more information on sending sponsored messages, see Sponsored messages