Well, I never use orkut, So its difficult to answer why people using Facebook instead of orkut. But I think Facebook will be more easy to start account, have some great services too.
ORKUT - Was actually INVITE only years ago.....they took to long to let regular people in, plus the design sucked
Orkut is almost dead now.............. It gives more importance to Integrate with Google+. Most profiles are order to link with Google+ profiles.
You can also try LinkedIn for professional networking and emerging social networks like Pheed.com, Medium.com etc for good social experience.....
Yes, we'll blame it on you. It just phased out, because Google moved onto bigger and better things. They sometimes tested various apps/online tools and then discontinued them. I still miss Sidewiki, and I missed Buzz until Facebook.