Personally, I prefer separate series for extras- not only as a creator, but also as a reader ( :
I creater extras for 6,75 to put world lore somewhere- it's way too long for tumblr or instagram, and I wanted readers to have easy access to it, without having to search for it at the end of the chapters. I also have mini comics there, and was holding event for ending of first volume there. Again, mini-comics are a bit too long for instagram (not so mini, huehue). Events and contests would probably be much easier to hold on tumblr or ig, but some of my active readers don't have an account there, and I didn't want to force them to create one in order to participate.
For the same reason, I prefer it to be in a separate series as reader. I check tapastic daily and it's already taking quite a lot of time, so I'd probably pass if I were to start using ig or tumblr again.
I used to use tumblr for work in progress and sketches, and it was pretty nice, but most people who saw it were not my readers- unless I linked it somewhere in the comment box with the next page CX
And, as a reader, I definitely prefer a separate series to wall posts, because at least I can comment without getting notifications about other people commenting for the next month : P