Pretty easy. The hard part would be getting users on it.

I was discussing this with someone earlier today. Twitter-like social networks require very large amounts of people to work relative to a site like SN. On SN we are all sharing 21 posts, like we are around a campfire as @dk says. This is valuable even for a handful of people. On Twitter we are all spread out and have to find each other individually; when there are more people it's easier to find someone you think is interesting, but hard if there are few people.