I believe you get out of a friendship, or any relationship, what you put into it. The capability to open up, to empathize, and to give without expecting something in return offers you the opportunity to make many friends.
I also believe that with our busy lives, we tend to compartmentalize friends. You may have friends at work, for example, that you wouldn't want to take to a ball game --- that's just not your shared experience. It doesn't lessen them as a friend, though.
Cyber friends are a recent phenomena, one we probably don't completely understand.
This community is unique in having a strong identification; we are all involved in baseball as/with a player, and that player is good enough to be able to consider progressing to the next level. And this is THE place where we can talk about that player, garner advice on the journey, and share experience. (Of course that blossoms out into other tangentially related areas, be it computer advice, humor, books, whatever.) But unlike our friends in the "real" world, there is the same level of commitment here without the potential jealousy, lack of experience, or just plain disinterest in the subject.
Because of that, our friends here fulfill an important role for most of us. And sometimes it's like the Stranger On A Plane. The guy you sit next to and you find yourself telling him things you wouldn't tell a close friend ---- because you can be totally honest with him and with yourself as you're never going to see him again!
Many of us visit this community daily; it takes a great deal of detachment to not feel the loss of a fellow regular.
And because of our shared Baseball Status, we identify strongly with each other. I think much of our response to these tragic losses is pure empathy.