i have a doubt related to back link. After posting, does the social site provide acklinks if yes then is it counted by google?

There are many methods and services provided by the SEO companies to get quality backlinks. Some of them are:
Blog commenting
Forum posting
Directory Submission
Blog posting and the list goes on...

bookmarking sites and forum sites are helps to get a good backlinks.

I would say directories , book marking are good way to get back links as it is under white hat method.

I think web directory gives us prefect back link.

Participate in forum discussion, blogging and blog comment are best and easy practice to get good backlinks.

You would need backlinks of different types. If you get backlinks from just social media, that will not do the trick for you. You need to get links by doing the following :
1) Gettting Press releases out often.
2) Contributing to forums like Reddit and Digg.
3) Social Media activities.
4) Writing blogs.
5) Working with online bloggers who have a good alexa rank to write about you.

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methods such as blog commenting, article directories, and rss directories I think these would help in making more backlinks...

the best way is great content , then many people introduce your site themself
i used this way for one of my site (SPAM LINK DELETED) and after some days i saw that i t get many link automatically , so i still belive that good content is the key

well ,for good baklink we must follow high pr website.It is most important for good backlink.

To get best backlink work with SEO techniques/method that would be helpful:P

I think it Forum dissuction and blog posting is a best way to get good back link.....

Commenting to blogs can also get you back links. Choose blogs realted to your niche and then start commenting. This blog post can help you

SNIP