I think it would be very useful. My only reservation is that the information would be public, which could prevent people from bookmarking posts on controversial/sensitive topics. But, it's a public blockchain, so I'm not sure how much of a problem that is (if at all). I wonder if it would be possible/useful to encrypt the author and permlink fields using the account's Memo key? But in that case I suppose it would probably cause the bookmarks to be lost if the user changes their master key/password?

Recently, I have been wondering about some more general questions about the "id" field in custom_json. Looking at recent transactions, I see, "follow", "notify", and "community", and looking at some older transactions I see that steemmonsters used to use things like: "sm_submit_team", "sm_find_match", or "sm_team_reveal". From my own testing, it seems like any text value can be used there - subject to whatever length and character constraints are imposed.

It makes sense to me that you use the "follow" id here, but in general, how are values selected for that field? If I were to use it for a new purpose, how would I choose a value? I assume we are free to make up our own values, but if so, how do we avoid collisions? And is there documentation somewhere to tie an id back to the application that's using it? I don't recall ever seeing anything like that.