From the Open Street Map Site (PDF):
On Sunday (6th Jan) the count of users registered on the OpenStreetMap.org website passed one million. As is typical of any such online community, this sign up count includes a “long
tail” of users who contributed very little or nothing at all, while a much smaller number of
users contribute significantly on a regular basis (more stats at http://wiki.osm.org/Stats) Nevertheless, many hands make light work, and with more than a million users now able to edit the map, OpenStreetMap’s coverage and detail is growing rapidly, and beginning to rival that of other map providers.
The process of contributing involves getting out and exploring the world. It can be a fun way to use smartphones, GPS receivers or other gadgets to collect data, though it can also be as simple as taking notes with pen and paper, or even just adding a local cafe or restaurant from your local knowledge.