BuddyPress, a plugin to make your site a social network, can be also used to bring more people into your giveaway. This integration is a premium one.
Showing all Giveaways a user has entered
With this integration, you will get a Giveaways tab under the profile page so people can see which giveaways has a user subscribed to. You can see if a giveaway is running and click on it to enter yourself.
Creating the Activity for each Giveaway sign-up
When a user signs up to a Giveaway, there will be an activity message added so all people following that one user will get the new activity. They will be able to click on the link of the giveaway and thus you will drive more people to your running giveaways.