Maybe Facebook users are being more self ware (privacy conscious) of who they add as a "friend"?
It's possible that today, students still do that, but other people don't, bringing the average down.
So, I think the answer here is that as the audience for Facebook has diversified, the average number of friends has settled to a more realistic number.
You can realistically only know a small number of people, and 300 is probably a good estimate for acquaintances rather than friends, over time you add people, remove people, fall out with people etc.
In the first few weeks you add more people than ever, then the curve slows down to 'old' and 'new' friends, in which case the trend slows down as all the people you used to know have added you.
I know that now, 5 years in to my use of an account, I delete more 'friends' than I add.
Not because of privacy concerns, mind you, I just realize now that a lot of the people I added in the early days weren't 'friends', they were just 'people' with whom I had some intangibly vague connection.
"Dunbar's number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships. These are relationships in which an individual knows who each person is, and how each person relates to every other person. Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group. No precise value has been proposed for Dunbar's number. It has been proposed to lie between 100 and 230, with a commonly used value of 150."