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.
Let's assume 25% of the USA population is on Facebook (2.2B of ~8B in the World apparently are, it's probably higher). So we have about 80M USA FB users. About 40/320 or 1/8 are in the 20-24 age group; but they're more likely to be represented on FB, so let's say 1:6 of USA FB users are 20-24.
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.
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...