Just share this Facebook status wall post:
There are still 7 copies of Dota 2 to be given away, so if only 7 people participates everyone gets a copy! If only 2 people participated both of them will get a copy while the remaining 5 will be put up for grabs again until there are no more remaining Dota 2 copies.
If there are more people, then only 7 will be chosen randomly.
All of this began with the Weekly Steam Games Giveaway: