What's the best source for City names?

Where do most startups get the city names from? We use Facebook for user registration and management. Facebook has its own id for locations. Some of the locations come in City/State format and some in City/Country format. I downloaded a dump from Geonames, but it is difficult to map these to Facebook city names. Also, GeoNames has London, UK and City of London, UK. It will be difficult for users to distinguish between the two. Same with Delhi, India and New Delhi, India. Users come from various sites and mention where they are going. This will come through a post string and user will not be able to select from a drop down list. It will be very important for us to get the city right. I wonder if anyone has a suggestion. Thanks Manas