You are looking to build a system that is a) as small and as simple as possible that yet b) still retains conceptual integrity.
You want the first because it's easier to build and get to market, and the second because it's not done till all the necessary parts are in.
Remember that the vast majority of your understanding of the problem space is not accurate, and will remain so until you encounter customers and users. So do as little work as possible; just enough to get you to those customers and users. Then you listen to them to find out the next parts to build. (The ideal case here is that you are also customer #1, incidentally.)
So no, not just some specific feature; you need the right coverage. But make sure you don't go overboard anywhere in particular, especially if it might not be needed.
How would you "factor in" ebay and twitter into one number that made any sense at all?
Thanks