Different between orkut and facebook... orkut sucks lol there is your difference. But seriously, in the broad aspect of it I do not see a big difference other than layout. Nothing "special" about either one of them really
Orkut is everyday worst. More difficult to add new friends. Everything you do is SPAM. If you try to create a new account in Orkut, you need to give to google your mobile phone number to receive a sms with a code.
Well, The biggest difference as per me, is That Facebook is built on latest Web 2.0 technology, giving you more features, apps, games, and real time chat, which wasn't there on orkut before facebook came into existence, and I doubt if they still have them, as I don't use orkut much. But, orkut is still leading in some countries like India and Africa probably ....
In user interface perspective FB is simple and orkut is little clumsy Dedicated ad spots in FB, in orkut ad is shown anywhere and anyplace