Manga about a survival game where people in the game make friends with outsiders using cell phones
I'm quite sure it is Real Account.

Color spread from the first chapter:



Summary of the first chapter (written by yours truly, since I can't find any with enough details):


  The story revolves around the social network Real Account. The protagonist generally didn't associate much with his friends at school, rather, he spent most of the time on his phone on Real Account, in which he had thousands of followers. When he was fiddling with his phone as usual, he (or rather his consciousness) got sucked into the virtual world of Real Account. This marked the start of the death game, where if you died in the virtual world, you and your followers would die in reality, and if your number of followers reached 0, you would died. The mastermind then announced the first game, where people were allowed to unfollow whoever got trapped in the virtual world within 3 minutes. Except for his sister, who told him she would never unfollow him, all followers of the protagonist unfollowed him.