The social media thing is a bit of a red herring. This isn't an episode about social media really, it's an episode about race and class - if there is a comment on social media in particular, it is about people's tendency to use social media to insulate themselves from their own awfulness, which they use social media to inculcate. It's an episode about people rather than about the tools they use.

The people of Finetime (and, it seems to suggest, the Homeworld – or at least, those people of the same class as those of Finetime) are a segregationist, hyper-privileged class that values appearance, propriety, and conformity with social expectations. They prefer surface-level friendships and interactions that enhance their social standing or self-regard to deeper or more complex relationships, and there's a blurring of social and parasocial relationships so that they're essentially the same. The Bubbles don't create this culture, but they allow their users to wallow in it completely and exclude anything distasteful or any challenges to this worldview.

Spurred to consider our own world, we might ask ourselves if there's any equivalency (again, the show's not an allegory, but it is a commentary). And if so, who is it that is performing the labor that keeps that class going? What does it mean to put them out of view? What do we become if we put everything unpleasant like that out of our minds? What happens when that becomes completely untenable?

The slugs are controlled by the Dots. The implication is that the people of Finetime are so loathsome that their own social media platform has grown to hate them enough to kill them (which, fair).

Who created the slugs? I dunno, a Doctor Who creature designer who liked The Macra Terror. Why didn't they just kill everyone at once? Because then there wouldn't be time for an episode of Doctor Who. If we want to guess at the alphabetical thing, a social media app presents users in alphabetical order, so for the rogue AI of the Dots, that's just what their programming supplies. Why don't the Dots just kill people themselves? If we want to guess, well, people would just take off their Dots if they started killing people (or whatever company produces them would see them malfunctioning and fix the error). And it seems a lot more obvious to people in Finetime when someone has trouble with their Dot than when a slug quietly nabs someone. But also, because the episode wouldn't really work if the Dots just sniped people. These are the sorts of questions that don't really matter –– personally, I don't think leaving their answers to the imagination weakens the episode at all, and putting it in just wastes time spelling out unimportant details. There's a certain degree of suspension of disbelief we need for stories to work.

Lastly, while it may seem sci-fi that an ultra-privileged, racially-homogenous class of people would create an environment for their children where they can succeed and meet no challenges while performing essentially no labor or doing anything of consequence, I invite you to visit the Hamptons or, like, any golf course.

There's quite a lot to think about in this extraordinary episode. In my mind, it's one of the best episodes since late Capaldi, and of post-2005 Who as a whole. Also always worth noting that Gatwa's final scene here was the first scene he shot as the Doctor (save for those with Tennant).