Definitely Twitter! But I would like to learn the secret behind using Stumble Upon. I've been Stumbled at least once and got an amazing amount of hit in one day but since then nothing really. I've tried but don't understand how to get people to Stumble your posts unless you beg them to.
Thanks Lori and everyone responding here - I am a relatively new blogger and had no idea how to really generate traffic for my blog, but I am going to work on twitter and get to grips with Stumbleupon, both if which I have accounts with but neither of which I really use or knew how to use.
Anyone interested can find me on Twitter at AgingMommy
You can link your Facebook page with Twitter so your every FB post goes on Twitter. Make life extremely easy. Also, check the http://ping.fm/ it's free and easy service that updates all your social networks at once.