If you want to remain fit and smart then sign up to Facebook and keep social networking site Twitter at an arms length.

Psychologist Tracy Alloway of Stirling University said some site like Facebook may improve working memory of the user as they require users to hold a lot of information in their heads, while sites such as Twitter and Youtube reduce users' ability to remember things.

Speaking on the topic at British Science Festival, Dr. Alloway said, "Twitter can cause harm because it produces a stream of information every second with no opportunity to process or manipulate that information."

Dr. Alloway blamed the surplus of technology for reducing peoples' need to remember things.

Facebook enhance peoples' memory because it compels them to make use of their memory cells more extensively to hold information.

Dr. Alloway further said video-war games and puzzles such as Sudoku improve memory.