Hi my question is that to get facebook likes, fb shares, comment, twitter retweets mean all social activity from addmefast.com and some other sites is really good idea or it will harm my site ranking?
If your site have lack of social signals like you mentioned above, addmefast will help you to drive them. Social signals are part of google ranking algorithm and it will help you to boost your site rankings. If you tired of grinding for addmefast points, check out this bot. It will help you to earn the points faster: http://wpglory.com/addmefast-bot/ Hope it helps!
I've used Add Me Fast quite a number of times. It certainly will help pad your social sharing numbers, but it won't do much so far as genuine engagement or interest go. Depends what your priorities are. So far as ranking goes, I don't think it will help or hurt much.