Personally I recommend facebook, twitter and lindedin as the most important social media. Facebook is an invaluable tool for marketing and website promotion. Twitter is an important tool for driving traffic to one's website. And linkedin is an important supplement for one's professional C.V. and for building one's professional reputation as an expert in one's field.
It is quite obvious from all the responses that Facebook and Twitter are the top 2. However, it all depends on where your customers are which social networks you should use. Furthermore, there are data out there that suggest that StumbleUpon brings in more traffic than Twitter. This would be worth taking into account also when making your decision.
Only few use it, for it is not that popular among the others. Why not? If you know that with leveraging Quora, you can gain more market then it is worth the try. If they have strict rules, then try to follow it.You can, if you want to.
I have tried all of the sites mentioned and I still like Facebook and Twitter better (by far). The other sites are a little "user-unfriendly"...
It's incredible just how much more effective Facebook is than MySpace nowadays. I have over 10,000 friends on MySpace. I have rough 500 friends on Facebook. I post a link on both sites at the same time. You'd be surprised to know which one gives me more traffic.