The big social networking sites like Facebook usually provide you with their mobile based website versions too, because there is a major security risk with logging through the main facebook login page while you are browsing from your mobile. The network on your mobile is unsecured, and this would be the most easiest way for hackers to hack into your facebook account, and steal your account information.
So, its always better and safe if you login to Facebook through its mobile version site if browsing from your mobile. Facebook has http://m.facebook.com/ for normal mobiles that run on Symbian or Java, and use a basic mobile web browser. And for iPhones and other touchscreen mobile phones, there is http://touch.facebook.com/ which you can use to browse facebook in a more secured and easier way.
Its a quicker way to login to facebook, the page size is reduced and also, the mobile pages are optimized in a way to provide the security to the users. As its always said, use what and where its provided and required. You have to use a full version facebook website always when you browse from your laptop or PC, and use the mobile version site when on your mobile browser.
The login sessions are saved, and you may automatically login from your mobile next time when you open the site from your mobile browser. And the facebook mobile version also has options to customize the homepage, customize the settings, add your phone number and many other things. All the notifications are displayed on the top.
Its the best way to login and access facebook from mobiles.