Regarding Yahoo answers
 

Hi friends,
How many of you have good experience (with respect to targeted traffic) via yahoo answers service.
Is it wise to buy some one who can post our links ?? Is it worth??
Any suggestions in this matter?

It was great in terms of traffic it brought me...

Since the links are�nofollow�, it shouldn�t be worth investing time for those who are purely looking it as a means to build links...(Although I have been reading that YA links are shown as �inlinks� in the Webmaster console)

But YA a great source of traffic. Helpful answers at a very high traffic site like Yahoo! Answers might get you some clicks from real people.

It might help you with with Yahoo ranking but not greatly! There are many other things you might try looking into like social bookmarking, networking, guest posting etc.