Two friends with a mutual friend who is obsessed with her waxing and waning boob size. The constant updates and bulletins regarding above-mentioned boobs are the cement and foundation of the relationship. Sometimes the boob friends may also become real friends, but the boob experience will always remain the core. Boob friends are sometimes also known as "bosom friends", which is an alternate definition of the more common usage of the phrase.
Friends by boob.
The boob friends occasionally play a drinking game known as "bosom shot" following every self-mention of the current boob size.