A person who is particularly annoying, someone you only talke to because there's nobody else around, and/or someone you only talk to so they don't feel bad.
None of my friends are in my art class, so I talk to Sarah. She's so boring, and such a default friend! I wish you were in that class, insert name of prefered friend
When your mom/spouse/friend leaves you alone in the room with a person you just met and it's getting way too awkward. Default friends are usually started if you know the person who left you is going to be gone for a while.
You start to talk; and gee, what do you know- a Default Friend has been made!
I left Barbara in the kitchen with my new friend Bobert while I went out to mow the lawn. By the time I was back, they were already exchanging numbers! What Default Friends.