i'm pretty sure they're more competent programmers you seem to think they are
i find it unlikely that followers are kept track of with just a text file of who follows who
well, information in a database still requires storage space. you can't really avoid that
each twitter user probably has some number associated with them, along with their name. they'd probably use those numbers for storing followers, because it'd use less space than their names
0-255 is easy with one byte, but you can't store 232 million people with those 256 numbers. to get that much you need at least three and a half bytes
that's the smallest possible way I can imagine to do it. and it wouldn't be future-proof. with that three and a half bytes you can only store up to 268 million, which has probably been exceeded by now, so I'm sure they just use four bytes (also because I don't think there's any way to use just half a byte), which gives them plenty
uhh. I think that's probably enough to get my point across