Why were some eaten and not others?
They came later in the alphabet - the plot was the ball internet/facebook thingy became intelligent hated the people - decided to somehow create the monsters and then killed them in alphabetical order - ignoring some to go in order of alphabetical killing list.
So survivors had names - not sure it if was surnames or first names -that came later in the alphabet - and because they were killed alphabetically it altered Doctor to fact it wasn't random but the floating metal ball.
Once the Doctor figured this out the metal ball though fuck it -so it decided to forgo the slugs and decided to try and kill main girl itself with kinetic energy - switched to hero boy when main girl pointed out he'd changed his name and actually his surname was earlier in the alphabet than her.
All her Facebook friends are white, I noticed, is that relevant?
Yes they were racist - refused to let the Doctor save them because they couldn't interact in person because he was not "one of them" - and it was his duty to save them as they were somehow superior - I think because of his skin colour as they seemed unaware he and Ruby were aliens to their world/society.
So basically we have young people on campus like area in their bubbles who are selfish arseholes who hate black people and are so incapable of thinking for themselves they need direction to walk and so self absorbed don't notice people being eaten in next office desk.
One of my kids has stopped watching now - I'll watch as long as other one still does. Next week look like alien Bridgerton.