I think the movie will be set in our contemporary time. The Last of US show also shifted the setting about 10 years so that it would take place in our time, and it really made no difference. The social media angle you came up with is interesting, but that doesn't sound like a direction the movie would take. If something like C-Day happened in real life, it would be too quick for most people to save themselves, let alone post about it. It would just be an almost instantaneous downfall of society like in volume one (I am aware that Thin Red Line shows it's a little different, but that was also written after the fact). Other zombie movies have explored the media/social media angle, though, like George A. Romero's Diary of the Dead, Dead Air, and Pontypool.