50 Cent is telling friends ... there's a reason he was laughing his ass off at Ray J and Fabolous -- their little Twitter feud was like a fight between two school girls.
We posted video of the Vegas "brawl," which shows Fiddy standing in the background grinning as Ray J and Fabolous went at it.
Sources close to Fiddy tell TMZ, contrary to what some witnesses are saying, the rapper didn't egg Ray J and Fabolous on. He just couldn't believe they were seriously arguing over Twitter.
Days after the fight, Ray J was still taking himself way too seriously in Vegas -- telling us, "I'm not that proud of nothing that happened."
Fact is ... rapper feuds over Twitter kinda ruins the whole street cred thing.
So we ask ...