how companies use social networking sites for marketing purpose?

[Nancy]

how companies do marketing on facebook ,twitter , linked in etc.

[Who, me?]

By posting/tweeting ads and links to promote their products:
Facebook has "fan pages" for products and compaines
Twitter lets them broadcast specials and news
LinkedIn lets them share news and post jobs
Foursquare lets them offer specials to certain people who use foursquare
The list goes on and on

[Amy]

Our company uses social networking extensively. Facebook and Twitter are good for making a following of people. Once you have a following you can saturate them with your promotions and ads. Because they are following you, they care about what you have to say, so they are receptive to what you have to say. I use socialmarketinglaunch.com to learn how to use social marketing in my business. I hope this helps!