What are some best social bookmarking sites for USA?

I know you guys can easily Google and get a number of social bookmarking websites on the go, but what keeps the list I mentioned here apart from the rest is the tried-and-tested factor. When you get those unknown websites from Google, there is no guarantee of the effective results. However, I have personally tried these platforms for my own business and I can claim that they are really worth.

Hope they prove substantial for yours too.