Edit: As user caladeniablue noted on Tumblr, favs are actually pretty different from kudos -- they're used more like bookmarks, but they're also public bookmarks that are right on your profile. So users might choose not to fav as many stories as they would kudos, either for privacy reasons or because they don't feel a need to revisit them.

TL;DR:

Works on AO3 and FFN tend to get more engagement on average than the works on Wattpad, but the most popular Wattpad works can get hundreds of times more reads and likes.

more reads and likes. On average, works on AO3 get a lot more lightweight rewards (kudos) than works on FFN (favs) or Wattpad (votes).

On average, FFN works get more reviews than AO3 works get comments.

It's worth noting that all of this is averaged across the whole platform in each case. Different fandoms -- and different ships or tropes or other subcategories -- may behave very differently in terms of how users engage, and how much they engage.

Edit:Several folks have pointed out that FFN favs behave more like bookmarks than kudos in most ways (and maybe also used to be harder to give?). I had already been planning to analyze bookmarks as a future step... I'll make sure to compare them to the fav data when I do!