Apart from yahoo and quora there are:
answers.com
allexperts.com
blurtit.com
Hello
Question answering is good activity in SEO. There are many websites for Q/A. Some are best
. Yahoo Answers
. Quora Answers
. Forums (Should be your niche specific)
With Q/A we can generate the high quality backlinks but make sure don't use the duplicate content. Use to write your own answers.
Lets Forum and SEO Mastering
You can also try Warriors or Quora, members on the said forum sites are active.
The Best Question and Answer sites are:-
quora, linkedin answer, forum, yahoo answer, is the best site