Asymmetric follow refers to a social network allowing many people to follow an individual or account without having to follow them back. It is also known as Asynchronous Follow or sometimes Asymmetric friendship.
An implementation of asymmetric follow can be seen in the use of Twitter When you see a social network where someone has 5000 followers and only follows 150 back – that's Asymmetric Follow.
In September 2010 Facebook started experimenting with a similar feature, which Facebook calls "Subscribe To."
References
Asymmetric follow Wikipedia(Text) CC BY-SA