What is a freemium policy in the context of a social networking site?

Freemium context in the social networking site is a pricing strategy basically free of charge when using Google +, Facebook, or Twitter in their terms of service. When the user wanted to have additional features of the networking site they are using, that is the time they will need to pay for the said feature. The freemium account is usually full of advertisement from the sponsor of the site. So, you cannot complain if advertisements will pop out or will be shown in your screen. If you don’t want these ads, you can always pay to remove those ads. That is when the premium account will come in.