bsterz - 11:40 pm on Dec 28, 2012 (gmt 0)
I can't say how much it directly increase earnings as I don't think it is a great for generating quick sales. However, it has definitely helped me in not only link building (a few really nice links), but relationship building (great tool to initiate interaction with) as well as generating traffic and increasing brand awareness. All without having to invest very much time.
I have seen others use less as customer service and more as brand management. When someone tweets negative things about them they send them a DM and try to fix things with this person and so on.