Multi person message scam
There has been a scammer trying to infect other people's computers. What they are doing is hacking a person's account and then messaging numerous people in a message from that hacked account. Somewhere in the message will be a link for you to click on. That link will infect your computer with a virus.
The message will have numerous participants in it. You probably will not recognize them. However, when you see the message the dangerous link may not be in the message. That is because Facebook discovered the scam and deleted the account or blocked the person. If you do see the link . . .
Do not click on the link!!!
If you think the message is legit, contact the person directly (outside of Facebook) that sent you the link and verify that they sent the link.
If you did click the link, Scan your system with your anti-virus software and anti-malware software like Malwarebytes