Dunbar outlines several levels of friendships and relationships in his work. According to his model, each person can maintain about five people in their support group of closest friends, about 15 people in a sympathy group who are close enough to confide in, about 50 close friends, about 150 casual friends and about 500 acquaintances.
Sounds about right. However, I have several friends in the second and third circles above who are online friends only, and I think our relationships are deep enough that they are truly close friends. I think this article is from the Obvious Dept. to some extent.