The development partner, need not necessarily be a "partner". You should give yourself considerable time to get to know someone before you take them in as partners. My suggestion would be to identify someone who can develop it for you even for a pay. And as you build the rapport & find yourself trusting that person more, you could engage as partners later.
The big positive in your case is that you already have a version out.
Your constraint seems to be finding time. Is it possible for you to pay & get the work done? It may be much easier to find specialists for specific features (say, mobile) than finding a partner straight away.
Go to conferences whenever time permits & interact with folks. Give yourself at least 3 months to find someone you would trust. Till such time consider hiring freelancers for specific work.
Just my views based on what I have experienced & read.
I really liked the advice by Mark Suster => http://www.bothsidesofthetable.com/2011/05/09/the-co-founder...
And this essay from pg => http://www.paulgraham.com/really.html
Good luck with your venture.