Diddy -- Possible Instagram Proposal to Longtime GF Cassie

File this under cheapest proposal ever -- Diddy (who's worth a reported $580 million) may have popped the question to his longtime GF Cassie ... on Instagram.

Granted, the rock is roughly the size of Pluto ... but rather than renting out a baseball stadium and proposing in front of an orchestra, Diddy posted a photo of the ring on Instagram for his 3.2 million followers.

Hours later, Keyshia Cole posted a photo of Cassie showing off the rock ... it's enormous ... but we still gotta ask ...