Depends, you mean as a pro-sport, or recreationally?
Recreationally (clublevel), it's very popular in many European countries, Korea, Japan, I guess the US as well.
As a pro-sport, Japan and Korea....not so much. France mainly I think in Europe, Russia, UK probably, maybe some other eastern European countries?
Japan and Korea seem to have a similar issue as my home-country (=Netherlands): many recreational players and great facilities everywhere and plenty of interest, but no top-players for some reason....
Edit: okay we did have a good stretch in the late 90's, with Krajicek, 2 or 3 other top-20/30 players and Eltingh/Haarhuis as a top doubles-team, but nowadays it's pretty bad...