Beyond the +1, Like and Tweet button, here are 3 ideas to sell or market your online service on social media platform.
1- Broadcast – The most basic way is to uses social media to broadcast your message and market your services. In this approach you establish a presence on a social media platform (a Facebook page, a twitter account or a company page on LinkedIn) and communicate about your service and content. Add a follow-us and a like button and you are set. This is mostly about retention and reaching your users on other platforms.
2- Engage – Another way to use social media is pro-actively engage users and start the conversation. It’s about finding who talks about what you business does and share thinks that you think may be of interest to them. Respond to their expressed reactions. For instance, you are selling cookies; send a message to someone when they are hungry. This approach will allow to increase your user base, but depending on how you to id, it is very time consuming.
3- Force – The most aggressive way I have seen uses of social media is on the services that will only work on top of them. They leverage the virality to the maximum, pushing the users to share events and actively seek participation of other users. Good examples are the so-called social games a-la Farmville and services such as bravenewtalents.com. This aggressive approach is very effective to increase dramatically your users base. It also requires quite a bit of thinking getting it right.
Those are just 3 ways I have seen frequently used. Each have their own level of complexity behind them of course.