Think about a MMO with 100 users in 1 room, the number of relationstips in that room is (n-1)^2. Add 100 more users walking around and you now need to send 4x as many location updates. You will quickly run out of bandwidth long before storing that much information is a problem.

Now consider a voting system you have 1,000 items and 1,000 users and you want to show how much people like them like each item? How well does that work when there are 100,000 users and 100,000 items?