The promise is for great things from these social media.
Currently I use Facebook exclusively on a personal level to keep in touch with my social colleagues and friends.
Linked In and Twitter I use at a professional level to create exposure for my business. I've been registered on Linked In for a while but only really started updating it over the last 3 months. The others are much more recent.
I enjoy the experience of adding updates and interacting even though I find some of the questions difficult to engage with.
I wonder what return is probable? Time will tell.