Yes, I think tracking something you already know about and are working on is very useful. Expenses, weight, time spent walking or working out, time spent working on specific projects etc., there are many things that can quite easily be turned into actually interesting graphs, even if it's just to motivate oneself, and certainly to get a more objective look at things or keep track of something over longer periods of time.

But number of tweets, or number of checkins per day? Nah.. or maybe it's a play on something Oscar Wilde wrote.. "To live is the rarest thing in the world. Most people exist, that is all." Not that I mean to be snarky, I assume it's up to the user what stats to display and hopefully how to display them; but to get me excited, it would have to be the "Piwik version" of itself, if you catch my drift.