Avira's blog reveals an interesting message which implies that your inbox might be full and you cannot receive any notifications. To sort the whole thing out you are asked to call a number that starts with 7 which means it comes from Russia.
Naturally this is some sort of trick launched by a cybervillain who will either ask you for personal information once you place the call or the number might be a premium-rate number which will cost you even more.
For your own safety follow the wise words of Data Security Expert Sorin Mustaca : “Facebook will never ask you to call any number. They will also never send you such a notification and definitely your Facebook Inbox will never get full. We strongly advise all users to never call any number present in such emails.”