Thanks to our friend Marshall Manson, and his interrogation of comScore data, we for the first time have a clear picture of Twitter usage usage in Europe, broken down by country:

Twitter usage in Europe

As he points out though:

It’s worth remembering that while these figures are a useful yard stick for Twitter usage and growth, they likely undercount both substantially owing to the number of Twitter users who interact with the platform via mobile devices and desktop applications like TweetDeck.

If these methods of accessing Twitter were included, I'd guess at somewhere between a 25% to 70% increase (based on these two data points - The Evolving Ecosystem and 40% of All Tweets Come From Mobile). And I imagine that would vary by market (for example that mobile use in Italy would be higher than the average).