Me a group of friends want to design a city building game for social networks like Facebook and I was wondering what art programs are used to create structure tiles lke the buildings, parks and houses. Is it 2D or 3D art?

I dont know which game your are currently looking at, but in general it can be 2D and 3D.

I think for those building-tiles, it is mostly plain 2D art, because it is faster/cheaper to create them. And 2d is enough if you don't need to rotate around it or have many animations (like characters). Program for this kind of art could be adobe photoshop (proprietary) or Gimp (free/open source).

If you are going to be making it for the social network setting you will want to have it be 2d for speed and simplicity. Most browsers cannot handle 3d reasonably. What many of the game groups do is generate the buildings in a 3d program to ensure measurements and perspective are correct. They then take a picture of it from the desired angle in an orthographic view. They then take the 2d picture (that is not textured) and color it in using Photoshop or similar programs. Hope that helps a bit. Good luck!