Oddbean new post about login | logout listening on every contact list update would be tons of data usage, this is why I've never done it. push notifications would be way more efficient. I'm surprised nostur is doing that without push.
from what I've been able to measure so far most data is still coming from images and video. I guess it may become a problem for very popular accounts but for those it also might not be that interesting anymore to know about new random followers, I will probably limit that to new followers that you already follow (following back)