Recently Mark Zuckerberg, founder of Facebook, updated his status on the social network site to “100 Million people on Facebook!”. No need for a press release or something fancy when all it takes is a one-liner status update to let the World know that the site you founded just reached a major milestone. That’s 100 million active users that are using Facebook regularly. This number tells us that Facebook is growing, and pretty rapidly too. It was about two years ago that MySpace hit their 100 million users, but at that time they included inactive users in the number as well.

When I first signed-up for Facebook, back when it was “The Facebook”, I wouldn’t have ever imagined that it would grow to be as large as it is today. Back then, I believe it was late 2004, Facebook was only available to those who had an email address from a certain list of colleges and universities. Granted, they continued to add schools pretty regularly, but not every college student had access. Now they have opened up to nearly everybody around the Globe which has really helped contribute to the 100 million milestone they recently reached.
Source: Mashable

Well, in the US MySpace still dominates, but elsewhere Facebook seems to be more popular, as most of the Facebook users are people outside of the US.
According to Alexa 36% of Facebook’s traffic is from the U.S., and in second place is the UK with 10%. So the U.S. accounting for over a third of all the traffic means it is pretty significant.
If by dominates you mean still holds a marginal user share that is not increasing, while Facebook quickly catches up – then yes.
Don’t forget the Toga party Facebook had to celebrate:
[valleywag.com]
Do you know how facebook defines an “active user” ? I’m curious about it.
I don’t know specifics, but I would take a guess that it means someone who has logged-in during a given period of time. What that time would be, I don’t know. It could be days or weeks.