Facebook relies on people using their service all the time, when they want. They want it to simply work. That's why they have this obsession about 24/7/365.25 uptime on their servers. It means also no friction when people use Facebook on their mobile, thus a 5 star app.
If you're being that precise the calendar actually has an average of 365.2425 days per year and not 365.25. In fact that was the major motivation for switching from the Julian Calendar a full 430 years ago.
I was just kind of surprised to see the .25. If you had put just 365 I'd have read that as "a year" and not give it a second thought. Instead it got me thinking if it actually rounded out to .25 or not so I went and checked. Apparently that wasn't too appreciated... :) No offense intended.
I hypothesise user engagement would suffer in the face of down time. This, plus the added scrutiny this technical failure would place on their stock, means now is not a time when they can be careless about reliability.
I disagree. There is so many social websites out there that you need your users to think you are the only option, If your service is down, people will have a look at others', and might them interesting.
Probably the main reason that they have so much traffic going to the mobile website (aside from international audiences with dumbphones) is that the app sucks. My colleagues use the webapp on their androids because they hate the native app.