Old app versions

Sometimes, the latest version of an app doesn't work on my operating system. Something similar happened with Facebook. I had to search using the mobile browser until I finally got one. Where can I get previous versions of some apps? Is there a site for this?

I don't think you can get previous versions of apps, I've looked in the past, and I have mistakenly updated an app where all it has done is added more adverts to the game. I had a look around at the time, and as far as I know it's not possible. If you find a way to go back a version, please let me know as I'd be interested.

Is it iphone or Android or Blackberry

A little Google search does wonders sometimes. Just search for old apps and you'll get what you want, it's the first result. I think they have mobile apps too.