What's a good cross platform messaging app?

My boyfriend lives in the US and I'm up here in Canada. To cut down costs of messaging each other we use a messaging app, but the app we're currently using (WhatsApp) isn't always sending messages. What's a better app for us to use? Not sure if it matters, but I have a Blackberry and he has an Android.

P.S. already tried Hookt, and it ran so slow on both our phones, so that one doesn't work either.