Issue with iMessages

Hey everyone!

I recently switched to a Nexus 4 (from an iPhone 4) yet I still use and love my iPad and MacBook Air. I'm a really active user of iMessages and because of that, a lot of my friends messages now skip my Nexus 4 altogether and go straight to my iPad or MacBook. As you can imagine, this is really annoying because I don't have my Apple products on hand to deal with iMessages.

Does anyone have an idea of how I can ensure when my friends can contact me through text?