You can send a message to anyone on Facebook, regardless of friend status or privacy settings. Strict filtering means that most of the messages in your main inbox will be from friends, while messages from non-friends are directed to your “Other” folder.
Under the last, the message will say "Seen " with a time and date, or, for group conversations, "Seen by " if they've been read. Nothing will appear if the latest message hasn't been read. If a friend has read your message, you'll see a miniature version of their profile picture.
When you send a message to someone you're friends with on Facebook: Your message gets delivered to their Facebook Chat list. If the person uses Messenger, your message will also be delivered to their Messenger app. Messages you send to people you're not Facebook friends with may arrive in their message requests.
It is testing a new 'Add Contact' request feature that allows users to chat with a contact on Messenger by merely adding that contact to the smartphone. This means that users will be able to chat with their contacts through Messenger without being friends on Facebook.
Filled Grey Circle with a White Tick This is the icon, on Facebook Messenger that shows your message has been delivered to the other end. However, you must understand that the word 'delivered' here, does not necessarily mean that the recipient has 'read' the message.
Fortunately, you can use Messenger without Facebook − but only if you have had a Facebook account in the past. Previously, it was possible to sign up for Facebook Messenger using your phone number and nothing else. But as of December 26 2019, you can't sign up for Messenger if you've never had a Facebook account.
Once the chat is open, you have to look at the small symbol that appears immediately after the message sent: If it appears the thumbnail of the person's photo, it means that the message has been read the message (and therefore has been ignored).
However, you can infer that you've been blocked on Messenger from the state of the status icon on a message you've sent. If you send a message to someone and the message is not delivered, meaning an unfilled check mark icon appears, you may have been blocked.
The recipient might have quickly read your message from the notification or status bar, but the app will not mark it as delivered or seen until the recipient actually opens the conversation with you. Or it's just that the recipient has deliberately ignored your message.
When you are using Facebook application, it will show you as “Active” on Messenger too and you won't be able to see the messages till you install Messenger. However when you are browsing Facebook web, it will show you online and will direct you to messenger when someone messages you.
Go to messenger App find (search) your friend in list and click on the friend name . so now in your app open one conversation in this section your app auto tell you your friend use Messenger are not, if not so send him app installation request and message also.
Similarly, if you want to know who blocked you on the Facebook app, it's at the top of your feed. A list of profiles and pages will come up. Toggle the results by clicking on People. If you've been blocked, their profile won't show up under this setting.
The most likely reason for this to occur is that the recipient has used the “Ignore messages” function within our chat. The marks that indicate the status of the sent messages will always remain in the status of sent but not delivered, that is, the circle with white background and blue mark.
If the iMessage never shows a “Delivered” or “Read” message, and it's still blue, then you may have been blocked – but not always. If the iMessage goes through and shows a “Delivered” message, then you have probably not been blocked.
In case of Ignore, the thread moves to Message Requests and disappears from the main Inbox as mentioned before. The new messages will no longer appear on the Messenger home screen.
The grey check mark is used in the FB Messenger app to indicate you've sent a message. Once that message has been delivered turns to blue, and once read, it changes to a photo of the person you've sent it to. 
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