There are 0 monthly active Facebook users showing on all applications. Fear not, it is being resolved. This tends to occur due to bugs in the MAU calculation. This is an issue that comes up from time to time, but it does not mean your traffic has dropped.

One point to note is that Monthly Active Users means Monthly Active Users. A drop in MAU's does not indicate a trend the day the numbers drop. Imagine the following scenario:

Day 1: 100 users
Activity 1: You improve the viral loop
Day 2: 150 users
Activity 2: Improve notifications
Day 3: 300 users

Fast forward 27 days and you happen to have 3000 users. But the next day you see numbers start to drop. The natural inclination is to feel that something happened in the day before that's causing the numbers to drop. The reality is that out of those 50 users you gained on day 2 from day 1, and the 150 users you gained from day 3 to day 2, not all came back the next day. Retention isn't 100%, but you don't notice the drop till 30 days after.