How would I define Social Media?
Before I started with this paper I only thought of Social Media in the form of Facebook. Which is not wrong, but I only thought of it as me chatting with family and friends on Facebook and keeping up with what they are doing. But Social Media is by far more evolved. Social media are a media for social interaction, using highly accessible and scalable communication techniques. Social Media use web base and mobile technologies to turn communication into interactive dialog.
Social Media is all about engagement; it is a two-way communication between you and the world. Leaving no barriers and includes everyone.
What is the “porous membrane” and how does this apply to an organisation?
The Porous membrane was first introduced by Hugh MacLeod (2005)
Social Media, and blogs have already begun to puncture this corporate membrane (sometimes from the inside out, but more often from the outside in. And this is beyond the control of the organization. What social media do is to create a direct channel of communication between employees and their customers, this is bypassing the traditional mechanisms the employers have put in place to ensure only selected members of the organisation are allowed to speak. What most of the employees havent realised is that they are the spokespersons for the organisation in the world of social media.
The four primary functions of social software: 
Communication – Blogs, Microbrogging, Location-based social networks and Social networking
Cooperation – Image and video sharing, social bookmarking and social cataloguing
Collaboration – Wikis
Connection – Social networking, tagging, syndication and mashups
studentmommyof3 
You posted: “What most of the employees havent realised is that they are the spokespersons for the organisation in the world of social media.” so, what are the implications of this?
Before social media there was only 1 way communication and the public only heard what the company wanted them to hear. (Through press release ect) These days with social media an employee can say anything they want to say, not fully aware of how they might affect a company. If employees start to notice that if they say things out of context on twitter / facebook they could potentially harm a company. This can go both ways.