Updated the first post!
New requirements: To join the free giveaway with less than 10 posts, you MUST send the 0,0001 BTC.
+1
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.
I will test this setting out, hopefully it will work this way. I'd feel bad to charge 50cent for a free giveaway!