If you're as big as Google, there's no such thing as a small product launch. So when Google introduced voice calls into its webmail service Gmail, essentially launching a Skype competitor, it was bound to be a popular feature.
How popular, exactly? Well, according to a tweet from Google, the users seem to love it, as more than one million calls were placed in the first 24 hours since the feature went live.
Let's put that into perspective. Gmail itself is huge, with more than 176 million users (according to comScore) as of December 2009, but voice calling is currently enabled only in the U.S. (*Update: Contrary to Google's announcement, the feature was also launched in Australia, as well as some other countries,) so it's reasonable to expect the number of users to grow as the service spreads to more countries.
The key word here, however, is "free": Calls to the U.S. and Canada are completely free (and will stay so for at least the rest of the year), while calls to other countries are billed at modest rates.