Updated the first post!
New requirements: To join the free giveaway with less than 10 posts, you MUST send the 0,0001 BTC.
But if I were you, I would change it a bit.
With less than 50 posts, you MUST send 0.001 BTC (~0.5$).
Don't know if 0.0001 BTC (0.05$) is enough to avoid exploiting. 100 shares from the giveaway would only cost 5$ with 0.0001 BTC.
This is a good idea. After 400 users signed for the free giveaway, you can raise the bar little bit higter, for instance, raising the posts to 50 posts and higher btc.