Also, I find that the more complex the work (if it has a +fame rating), the more fame they get from it. Simstagram takes a few secs, so the fame count goes up by almost nothing. My developer sim, however, worked on her new game for two-three days, as it builds so, so slowly, but she did get a one-star fame rating out of it! So now she's finally on her way to becoming the famous dev she was always meant to be :naughty:
Same for posting videos you took a while to make and edit and add transitions to, they also give you more fame than simstagram, etc. So as far as I can tell, the complexity of the task partially defines how much fame you get from it :)
(But truth be told, because it takes quite a bit of time and effort to raise fame, and because some of my sims should've been famous ages ago, I just gave them up to 3 stars for "free" (because, let's face it, my well-published archaeologist who's married to the previous owner of one of my most successful restaurants, spent half her life in the jungle and even more time working for the museum should've been famous a long time ago :) I don't play her that actively though).)