Nice concept - it looks pretty similar to Paywithatweet.com which (I've heard) does actually get people tweeting about the product.

The drawback with these sorts of concepts is that it means the product is shared before people actually use it, which makes the endorsement seem a bit forced. Having said that, people do use it to get access so maybe that doesn't put many people off.

The design looks great by the way, I didn't notice it was Bootstrap until jaryd mentioned it.