Nik,
It's just 18 hours ago when I asked you to locate the add function of the OssnNotifications class.
I didn't do that to steal your time, I did it intentionally because the expected parameters are clearly documented there by Arsalan.

And now you ask: Please let me know which parameters to use here. ?!?

Sorry, but if you're not even showing the slightest will and interest to understand things, then please try getting help somewhere else. At least I'm no longer available on this subject.

So if its correct. The next step is to add notifcations to ossnnotifications database right? Please let me know which parameters to use here. $notifications->add('mention:post:created', $params['ownerguid'], $params['subject_guid'], $user->guid);

He is right you used correct $args['text'].

Sorry Arsalan. Earlier he told me that $post is wrong . Also he told me to use text element of args array. So I thought $args['text']. Since he told me to change pregmatchall line, I thought I should replace $post with $args['text']. Please guide me .

Sorry Nikhil, no offense here but I begin to think that you are really not sure what you are trying to achieve. Despite we telling you again and again. Z-men already told you what to use.

Is it args['text']?

Yes, both are be usable. But since the string in description is saved in a JSON encoded format and would need an extra line to decode it first, I would prefer using the much simpler text element.
So please change the preg_match_all line to use that text element of the $args array

Yeah I commented it for now. Iam assuming the field(element) as either text or description .Am I correct?

That's correct!
So,
1. you now know which parameters this method is actually expecting
2. you now know that this method would add a new record to the ossn_notifications database table, thus passing any wrong parameters would result in a bad/unusable record.
So, before you are really sure about which parameters to be passed, I strongly recommend to temporarely comment out that line in your code like

// $notifications->add('mention:post:created', $params['owner_guid'].........,

Done that? Okay.

Then back to the not working preg_match_all line:
As your debug output at the bottom already shows, this function's second argument has to be a STRING. A string which will be parsed for one or more occurences of @user1, @user2, and so on.
But you're not delivering a string, but $post. And that's NOT a string but an object.
So have look into your debug output of the $args array again, and find the element which contains the string you're interested in.

Its OssnNotifications.php right? Theres an add function present in that file