We totally go for quality now. But we tried both.
We tried full on 'social outreach' for a while as a 'quantity' approach. The results were OK, but we were all over the place with no follow-up.
Then we wanted to try 'quality' where we cherry-picked the forum questions, social network posts, blog posts, etc, that were perfect for our input and participation -- and the results are much better without driving us up the wall.
Now we have time to follow up on each reply, or piece of input we make, and that delivers much better value as a whole -- and producing value, is where it's at. If you produce real value, you'll always see results in the bottom line.
I go for quality. One main reason for it is the cost of supporting many networks, I believe it is not proportional to the income, i. e., adding more networks will increase your cost and dilute your gains. It's like the 80/20 rule, going for the second-line networks will add too much effort for the return.
This doesn't mean that the quantity strategy is wrong. This only means that for me, It's the best strategy.
Tatiana, I agree with Fernando. Quality will always reflect better on you and your business than quantity. If you are too prolific, you may lose the impact of what you want to get across. On the other hand, if you don't stay active enough, people may lose interest. It's a fine line between the two.
Assuming you are wanting to use social media for a small business start-up, you may want to incorporate something like weekly specials, or a discount code every now and then. This will help you gain a following.
Best of luck to you!