Diddy Proposes To Cassie On Instagram

File this under cheapest proposal ever -- Diddy (who's worth a reported $580 million) 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.

Along with the photo, he wrote the caption: "Baby do you like it? I just want to get you wat you Like! I just wanna Mk you smile :)" Cassie (who's been dating Diddy since 2007) responded, "It's perfect!"

Hours later, Keyshia Cole posted a photo of Cassie showing off the rock.

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