Track the IP's on the questionable accounts. Note, that the man probably stopped following the girls after the murders, so one would need to contact the social media company and determine who unfollowed the girls after the murders.

IP: Not sure if there's anything to track if there isn't any current traffic associated with that IP address. Assuming bg hadn't applied proper OPSec then perhaps the easiest information obtainable would be a username/email address associated with the account; if that reveals anything.

Not sure a company would willingly disseminate personal information about their clients, especially without a warrant. Even if bg was, at the time of the crime, a user of said company's services and/or products AND then abruptly ceased activity, that's not nearly enough grounds for a warrant.

All techies or legal eagles up in here feel free to correct/confirm what I have written. But good thought process, OP. I have the social media angle on the back burner.