I find that twitter in general generates less likes than instagram - everything swooshes past you at immense speed, and your post is drowned out and disappears. Twitter is extremely fleeting. It's in the nature of the site, twitter is mainly text (which is easily produced), instagram requires pictures (and you can post more hashtags). The main advantage of instagram is that you effectively have a gallery, so even if you didn't get any likes with your initial posting, the art is still there to be seen later by new visitors and it contributes to an attractive profile. It's not as easy to browse through posts with twitter IMO.
@ndominiq : As the others said, if your activity drops off, so will the watchers'. Keep posting regularly, choose hashtags wisely, and be an active part of the community. Some go around and follow/like everyone even vaguely related to what they're doing in order to increase their reach, others stick to only liking and following that which they actually like. Which method you decide to go with is up to you, but in the end the most important part is that you create new content regularly, so that there is a good reason to follow you. (;