Sort:  

Proving that there are genuine ones is easy; finding one is difficult.

By the way, why only (at least) 10% chance of winning?

The chances of you winning is just too slim. Imagine, a giveaway to only 10 Lucky winners where there are 'bout 1.5k+ registered for the giveaway. Check for instance, the recently concluded $Ghost token giveaway on Twitter.

Well, if ever I would join any giveaway online, I would only provide an anonymous email address- nothing more.