I disagree that Likes had that "clear, simple use" case. I for one didn't use it as a bookmark, simply because I used it so often for the other case you cite: "

I disagree that Likes had that "clear, simple use" case. I for one didn't use it as a bookmark, simply because I used it so often for the other case you cite: "telling the poster that you liked it" -- including for tweets that were relatively trivial: e.g. "I appreciate that you replied to my tweet". I follow several thousand users so I guess the timeline algorithm doesn't bother showing me Liked activity. However, I do see "[some user] and 5 others liked this tweet" when I do a search for tweets, and I find that note to be more helpful than not, to be honest.