I'd say you have to measure your cost verses benefit, with the primary cost being your time.
Honestly, had you not been here on the forums, I would not have found your comic. But I've never seen you on twitter or facebook and I don't follow tumblr or instagram, so I don't know about those. I'd say if you aren't seeing a return on your investment (again, your time) on a particular platform, abandon it. Try a different platform. I think there was a really good post about the cost/benefits of each platform and how long it takes to get a following on each, but I can't find it right now...
Personally, each platform brings a different crowd for me. On facebook, my immediate friends and family who don't have twitter or tapas accounts, this is how they keep up with the comic and my other work. On twitter I connected to alot of other creators on #comicbookhour and got a huge surge of followers that way. Now I use twitter for status updates, since the whole "post on your board here on tapas" is completely bogus and I think it annoys most of my followers honestly.
So, do just what works for you.