You can offer it on a package basis or on an hourly rate. You can offer social media, or offer it in separate services like Twitter or Facebook etc. You can log into your clients twitter account on a daily basis or every other day and set up tweets ahead of time, interact on behalf of your client with like minded businesses on twitter, the same with Facebook, Linked In etc. You could offer a package for social medial including all 3 the most popular social media platforms which are Facebook, Twitter and Linked In.
Today a lot of company's/businesses networking is done by joining groups within Facebook, Linked In etc. Being part of forums online. You network and interact with people and build up friendships and relationships. Networking is not about selling your project, its about building relationships with likeminded people in the same business as you and maybe business can come from this.
It is a great service to offer clients as its a much in demand service today. People cannot do business today without belonging somehow to social media.