Maybe people have reached what could be considered their 'critical mass' in respect to the number of friends.
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."
Facebook is probably showing you updates of probably around 120 - 150 of active people, with about 7-20 very active close friends. And as someone has already mentioned, this has been proven against Dunbar's Number, and in other studies.