This article does a very good job of re-working a concept from economics to apply to social media. It has caused me to think about where I am spending my effort in social media, and has also made me wonder whether there is a third level beyond "Stock", which is more durable artifacts (like books, podcasts, music, artworks etc.).
For my wife, who runs a glass business, the split would look like this:

Flow: Twitter, Facebook
Stock: RainpebblesGlass blog and website (currently under construction)
Durables: The fused glass objects that Rainpebbles sells

For myself, it is slightly more complicated:

Flow: Twitter, Facebook
Stock: This blog and various other blogs
Durables: My nascent book project