backlinks from social media

If the backlinks would come from actual users, then it would be good but if it comes from fake and spam accounts, nothing good would come from them.

tell me, how can google differ between fake and real account? they can NOT

It is all good until you are dripfeeding and diversifying it well . I have never seen a site getting deranked for blasting social signals .

if you think about buying social signals try to find a service that has all social media sites. Not only g+,fb, twitter and maybe pinterest.
That won't look natural. And the amount of social backlinks should be spreaded naturall. Means for example fb 100,tw 100, g+75, delicious 10, reddit 10 etc....
It does not look natural if you have 100 social backlinks from each.