Cristiano Ronaldo Slid Into Model's DMs, Not In Relationship

It goes down in the DM ... at least, that's how it went down with Cristiano Ronaldo, who social media pimped his way into a hangout session with Casi Davis, TMZ Sports has learned.

Our sources tell us ... Ronaldo has known Davis for a while and DMd her -- along with a bunch of other attractive ladies -- to meet up while he was in Miami.

We're told Ronaldo spent time with a couple different ladies during his trip to MIA ... but clearly, there was something special about Davis ... who he was kissing on (and more) poolside on Sunday.

We're told ... there's nothing serious between the two -- they're just casual friends ... and now it's onto the next one!