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.