Ask HN: any good sites for running sweepstakes, and general advice?

Been researching and looking into running a few contests or sweepstakes to get some attention and signups for my startup. Ideally I would want to run it across multiple social media channels, have a nice hosted launch page w/ good SEO, and make the legal, management, and selection of winners easy as possible.

I have come across a few platforms that help with this, such as : http://www.wildfireapp.com/, http://onekontest.com/, and http://www.strutta.com/

I'm just basically looking for any insights into running sweepstakes to generate signups/visibility and the best way to do it with minimal effort. Thanks!