The social media star -- who's still doing his thing online after Vine collapsed years ago -- was arrested and booked into jail in Mecklenburg County Friday after a few arrest warrants were issued there to haul him in all the way from Cali, where's he got a home in Encino.

Per the warrants, obtained by TMZ, a judge found probable cause to sic the hounds on him based on preliminary evidence Charlotte police submitted over an attack this past Monday for which they believe Grier is responsible -- and which left the victim seriously injured.

The warrants don't get into the full narrative over what exactly happened, but there's enough there to get a sense of how badly hurt the victim -- William Markolf -- was, allegedly at the hands of Grier ... who allegedly assaulted Markolf and then jacked his $1,200 phone.

The alleged attack is said to have left Markolf with injuries that cops say included a broken left orbital bone (eyeball socket), brain damage, bruised ribs, bleeding from the back of the head, loss of hearing and more. In other words, they're saying it was a major beatdown.

Unclear what exactly led up to this ... but in any case, Grier is now out on bail. Grier was was booked for Common Law Robbery, Felony conspiracy and Assault Serious Bodily Injury ... and also posed for this mugshot on his way in.

There's another person named Luke Foushee that the police claim Grier conspired with to commit this alleged crime -- but it doesn't appear he's been arrested just yet.

Grier's teenager Vine fame has carried over to IG and YouTube of late, where he's got scores of followers and subscribers ... and continues to churn out content as an adult. He's also famous for being the then-youngest contestant on 'DWTS' in 2015, when he was 15.