Advice on setting up a competition for my software launch

I want to setup a competition on facebook (and twitter-pinterest) to launch my new software, the prize will be free copies of the software.

Im just wondering if you guys have any clever ideas for getting the competition to go viral and targeted towards the right people? (wordrpress users)

Any suggestions would be great

I know its quite a open ended question but would love to hear some of your advice

Thanks Guys