I've been doing thank you posts for awhile, they're pretty helpful when you're just starting out because it helps you make a more direct connection with your readers, early on I got into some friendly one-on-one chats with some of my readers because of the thank you posts, I even got some good feedback sometimes. The other thing that's helpful is, if another creator follows you posting on their wall can be a little bit of a signal boost for you, I would also use it as an opportunity to thank those creators directly for their support and comment on how much I liked their work. Later on I was able to go back to some of those creators who I had started a dialogue with (many of which I had continued to chat with outside of the thank you post) and ask for guest comics (I ended up getting 9 of them!!!).
So yeah, thank you posts are great. They may not be the most effective way to grow an audience, but they're useful for connecting with your readers directly and building up a little bit of a community.
All of that said I'm going to stop doing them once I hit 500 subscribers and start doing something else, because I feel like I've outgrown them, and when I post I tend to get a decent number of new subscribers so it's much more time consuming than it was when I first started and I only had to make 2-6 posts in a week.