Will Foursquare will have 1 million members signed up in the next couple of week? You bet. This tweet was just published and they are excited to have 1M Checkins in a single day.
Why does this matter to you? If you are a business owner you need to be on Foursquare. People will help you advertise. The best way to describe the service is to actually quote directly from the Foursquare site:
I am compassionate about helping others with online and offline business concepts. While providing training and consulting for Social Media I also enjoy volunteer work. I've owned my own businesses since 2007. Networking both offline and online is a joy. Please connect with me and let me know what you are working on too.