Post on your (user) behalf

Apart from making it easy to register, the other main point of social login is that we can use it to spread the word by posting user’s website activity on their timeline…

So how can we enable it so
This app can get permission to and then actually “Post on your behalf”

e.g. every time a user registers with their social media account I want to post on their time line:
<username> is now doing XYZ on my website (e.g. bought something or commented) and include a logo/graphic

http://ww.wp.xz.cn/plugins/wordpress-social-login/