I'm planning to host a Wordpress Blog on AppFog. As i'm novice and their documentation seems to be lacking details, i want to know that if my Data will be preserved if any instance goes down ?
They don't have a persistent filesystem yet, so it not only MAY, but rather WILL go down at some point. You'd better use a remote repository and host it on your local machine, and update your wordpress from there. Right now, AppFog is quite different from cpanel's hosting you're most probably used to. The wp.stackoverflow.com should have docs on git and AppFog, start there.