I swear any website that does giveaways get a huge spike of attention while it's on, even unpopular ones. I think you should've made a rule to prevent brand new users from joining, and give a certain timeframe of being a temper as a minimum.
We've done plenty of giveaways for existing members before. Sometimes it's necessary to try and drive new visitors to your site in order to grow and giveaways are a sure fire thing. We can only hope that some newcomers or lurkers may decide to engage more once it's over
Giveaways can surely bring new visitors, but they don't make them stay after that. But I guess it's still a good thing, you'll be getting more ad revenue I presume and other things.