Personally, I find these emails annoying. I don't doubt that they work in general, though.

Great points rishi. Facebook can teach us all a lesson in social networking and how to improve our own web 2.0 products.

Beware, a strategy like this might aswell backlash. Maybe it would not be as easy to get away with if you're not the 4th largest site in the world.

wow! keep with the good work, captain obvious. sure they do!!! that´s why they grew that big! if a regular site sends you that crap, it is spam... if your pals send it, you cannot just say no.