I think Facebook = Social networking Twitter = Social + professional networking So, its difficult to compare them.
I agree with web hunk Difficult to compare because they offer different things With FB you hook up with friends and with Twitter you check what famous people and businesses are doing They both offer different things and there is space on the web for them both
You can compare: http://www.alexa.com/data/details/traffic_details/facebook.com http://www.alexa.com/data/details/traffic_details/twitter.com
your comparing two different social networks, it's like which is better, apples or oranges Facebook and Twitter both have their good traits
Facebook is more popular simply because it has been in existence for long! Twitter too has been doing fairly well lately, in climbing the popularity chart! Regards, RightMan
Facebook is more popular, but they are different things. Facebook is for keeping in touch with friends and social stuff and Twitter is for spamming people with links to your blog.