Foursquare — and the general trend of geo-location services — present a huge opportunity for campaigns. See where your volunteers are. Share where your candidate is. Let volunteers see where voters are. Find polling places. Foursquare doesn’t do all of these things, but it is quickly becoming so widely adopted that if it won’t soon, someone else will.

TechCrunch reports that Foursquare has now passed 2 million users:

It appears that Foursquare has just crossed the 2 million users mark this morning. The location based social network has been growing fast, adding 100,000 users per week. Only three months ago, Foursquare passed one million users after taking a year to accumulate one million members.

TechCrunch also notes that other services have equal, or larger, audiences already:

It’s important to note that other competitors have already crossed this mark. MyTown, another location-based network hit that number in May, Brightkite hit 2 million users in February. And, Loopt just passed 4 million users.