Splitwise was built specifically to keep track of IOUs with other people. All bills on Splitwise are split – you can't add an expense unless you're sharing it with someone else (someone who you owe, or who owes you). Splitwise is only useful if you have friends to split expenses with.
In addition, we think it's extremely important that everyone involved in a bill can log on and see that bill. Splitwise is a communication tool to help everyone stay on the same page. We believe that keeping a shared record of what has occurred is the best way to avoid stress and keep things fair, which is our mission. Everyone should have a right to see what's going on, fix problems, and add their own expenses – otherwise, it's very easy for people keeping separate records to start disagreeing on how much they owe each other.
We realize this is a lot to ask, and that some people don't want to invite their friends right away (or at all). We encourage you to play with the interface a bit before adding your first friend, or to add "email@example.com" as an example friend to play around with (adding friends at example.com won't send spam emails to anyone).
However, if you're unwilling to eventually add your real friends, then Splitwise probably isn't the right service for you. There are a number of great personal IOU apps available on the web, iPhone, and Android – go try 'em out and find the one that's best for you!