This evening Twitter CEO Evan Williams put up an interesting post about Twitter mobile usage. By just about every measurable metric, it seems to be skyrocketing. He also included a graph of the top 10 ways people are now using Twitter. This includes both Twitter’s own apps and third-party clients, but notably, Twitter for Android is nowhere to be seen.

I’ve confirmed with Twitter that this isn’t a mistake. It seems that Twitter for Android is in fact not in the top 10 ways people interact with the service. That’s incredible considering that Twitter for iPhone is number 4 and Twitter for Blackberry is number 5. Both of those were built by the Twitter team, just as the Android app was.

Even crazier, it’s behind third party clients TwitPic, TweetDeck, Echofon, UberTwitter, and even Google Friend Connect!

Obviously, Twitter downplays that in the post (and by downplays, I mean, doesn’t mention it), and instead only mentions Twitter for Android in the intro to say that it “launched a new version this week.” Remember, “new” is the best tactic to get people to try something.

So why is next to no one using Twitter for Android? Twitter won’t say. But I suspect it may have to do with the awful experience of trying to find apps in Google’s Android Market. Twitter for Android is by far and away the best client for the platform — but you’ll note that none of the other popular Android Twitter clients made the cut either.

So perhaps Android users just don’t like using Twitter? That’s probably not the case. Remember too that a lot of the newer Android phones come with custom skins that often include Twitter functionality without the need for a separate app. Still, for browsing tweets, Twitter for Android is much, much better. People should be using it, but they’re apparently not.

Update: One other thing undoubtedly at play here: Twitter for Android only works on Android 2.1 and later. Again, that’s largely an Android issue as fragmentation is preventing people from upgrading.

More: Twitter Now Over 145 Million Users, Almost 300,000 Apps