How to advertise my sites and blogs via social networking sites?......

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I have more one websites which are having useful contents for all the people... I have also few more blogs to read and get some useful needs of them.... Here onwards, now i want to advertise all my sites and blogs in some famous social networking websites like twitter, facebook etc.. But i don't know how to advertise my sites through social networking sites... On the guidance of my friends i have already opened my free accounts in those social media sites.... Still i am not getting an idea and method of advertising my sites in them.... Please let me know the ways to advertise my sites by giving some steps in detail....