Facebook users may create a personal profile, add other users as friends and exchange messages, including automatic notifications when they update their profile. You may also join common interest user groups. Facebook offers Photo application to upload albums, notes where you can blog, and games you can be entertained with. Twitter offers micro-blogging services, enabling its users to send and read tweets. It revolves around the principle of followers. When you choose to follow another user, that user's tweets appear on your page. You'll see a mix of tweets scrolling down the page dependent on the number of the people you follow. Each social networking site has their own individual function and style.
facebook is a place where you can socialize,talk with people,chat,share videos and photos and etc, but twitter has only limited activities.
fb is social networking and twitting is just giving information to ur followers.. by simple one tweet/pic.
Twitter though is a socializing tool, doesn't require that much of your itme to remain in touch with everyone else. If you are going o use it for business purposes, then initially you have to get a lots of followers and once you are through that phase it is just a minutes task to spread your word.