As social Marketing provide us the link back to our sites... But I have a question here how Google come to know about the quality of links as in this case google can find only Quantity of links.
What do you understand by quality of a link? If you know answer of this question than you can easily find out the answer of your question yourself.
Google considers both quality and quantity,you will only submit your links to most popular sites and if they are popular then they are quality back links for your site. Hope you get what you wanted.
As per my knowledge, If links are related to your niche , Google count as it as a Qulaity Link... But if I submit my site to Bookmarking sites that include all subjects.. then how google measure the quality link
A link coming from a quality and popular website is also counted as a quality link (irrespective to the niche... What if my website will be linked from whitehouse.gov(as it is not of my niche), what you think Google won't consider it as a quality link??