This will be a good blog topic when you get the correct answer. 

Personally, I would not send mass messages on Facebook. 

I think there is no way to guarantee all your friends will see your post. There are too many filters and setting that each individual has set up and it driven by the algorithms and usage.

No idea Sussie, hopefully someone here will know.

don't do FB so I really don't know.

Facebook?  Just post.  You can't guarantee anything on there, depending on how people have their feeds setup.

On AR the tagging only works for one person at a time.

maybe Anita Clark might know this answer...or Nestor & Katerina Gasset (katerina)....

I believe that at one time it was possible but now you have to pay to do it. I don't do a lot on fb but this is what i have read.

If several people have commented on my FB page, you can privately message a group by a checkmark next to their name when you send the message. But I like to personally send messages instead of one big message.

I have never done this.

I can't imagine a need to mass message on Facebook. The number of tags is limited, but I would suggest care in this endeavor. 

Sorry Sussie, I am not help.  I am not a big user of FB ... don't spend much time there at all. 

Only know private msg on FB and often people do not even look.

I don't mass message on FB but you can tag multiple friends.

I hope you got your answer, as never tried to send a message to all my FB friends.