My ipad2 couldn't log in or sign in to any web page

My ipad2 couldn't log in or sign in to any web page.

Example, in the facebook login page. After I type my username and password correctly,

they keep turning the page back to the login page before I sign in.

And this situation happened in any other webpages like hotmail, yahoo, whatever webpages which need to sign in.

Thank you very much if you helping me by providing some ideas, thanks