That a great thing to think ? Are you share on facebook or twitter ? I share on facebook most of the time. Most of the wordpress user using share this , sexy bookmarks that all include facebook and twitter. You should read this post

http://www.point-star.net/2009/sharing-on-facebook-vs-twitter/

Facebook have many user , twitter have many share user and retweet user. So, I should share facebook and twitter at the same time.