[shajith]

I don't use Facebook as much, but I follow a similar strategy for other things, like GMail. Instead of a different browser, I set up a 'site-specific-browser', like Fluid[1], for each such website.

Aside from the benefit of not staying logged in to your email/FB/Twitter when browsing random websites[2], this also lets you treat constantly open web-sites as legitimate apps in your switcher, frees up those permanent tabs on your browser, and provides for features like an unread count badge on the application icon when you switch apps.

[1] http://fluidapp.com/

[2] Obviously, this means that features that rely on you being constantly logged in to FB/Twitter (those Like and Tweet buttons) won't work.