How many of these users simply aged out of the demographic? I wouldn't be surprised if it's just a combination of people aging out and not too many new users signing up.
How many of the "users under 25" simply turned 26?
It's like an old article I saw somewhere about teen pregnancy dropping heavily once they reach 20 years old...
So, back of the napkin we can expect around 13M/5 or ~2.5M Facebook users to age out of the under-25 age-group in USA in a year. It's around the same ball-park.
Disclaimer: I'm recovering from flu.