I would suggest trying to increase the overall level of engagement with each individual post. How Facebook works it that you need to have a high level of engagement with the initial group your post is exposed to otherwise it won't push it out to all 700 of your fans. So I would assume that a majority of your fans aren't actually seeing your updates.
You should try and do less broad stroke marketing and try to focus down on who your core audience would be and finding out how to best get your work in front of them. This is approach is more relevant to you because you're going after sales - it takes a lot to get some one to pull their wallet out but the easiest way is to deliver something that they're looking for.
Listing sites are sort of a tough market. I think listing work (pdfs or ebooks) on Amazon is actually a lot easier than Comixology. You can sort of game the system by adding your own book to your cart along with other books that you think are in the same genre that get more sales than you. This increases the likelihood of your work being recommended to a buyer when those other works get sold.
Also, with Amazon, you can usually offer fire sales or sell portions of the book for free.
Just some random thoughts on the topic, haha.