Driftwood is a partial erection, or a half flaccid/half erect phallus. This is commonly acquired while one is interrupted during sexual activity. It can be confused with a "Broner," however, it is not the same.
The guy, or guys, that you meet and have sex with while you're at the beach. They, and their "wood", drift in and out of your life for the day/night, the week, the summer - however long you're in a beach environment.
When a man starts spending less time with the bros: this is generally caused by his hope of "getting some" with his new girlfriend.
Mark: "Hey man, have you seen John lately since he started dating that new chick?"
Paul: "No i haven't... i think he may have become a victim of The Driftwood Effect."