This Google Trends graph comparing photo printing sites is interesting. The seasonal Christmas surge is very evident. It also looks as if by this measure that Snapfish has caught up with and pulled ahead of both Kodak and Shutterfly.
Key: snapfish, kodak easyshare, shutterfly, walgreens photo, walmart photo
Probably however the Snapfish versus Shutterly comparison is the only really valid one here because there is an obvious search term that people might use for both of those. For the other services there are likely to be other search terms that people used over the years.
(Disclaimer, I work for HP of which Snapfish is a part.)
Update 2019-09-14: The above links no longer work.This is what it looks like now. Back then Snapshot was a market leader – now it has declined precipitously.