In sites like facebook or orkut or friendster what's displayed at the bottom instead of main web address like static.ak.fbcdn.net or profile.ak.fbcdn.net? Are these different servers or they are just used to misguide the hackers?
They're different servers that are optimized to deliver particular types of content. For example, if you can segregate all the static content from the dynamic content, you can use a very fast specialized webserver to deliver the static content which can assume that all it does is just serve up data from files, but which can't write to any of them (in fact, you can configure the server so that it can't open any file at all for writing or overwrite any part of the program, making the server extremely hardened against attacks). Looking at the URL, I'd guess that the data is actually being served from a Content Delivery Network as well (i.e., a geographically distributed collection of identical servers).
However, it should be noted that none of that will fool hackers. Or anyone else with web development tools. The purpose is to optimize servers for delivering particular types of content, with particular risk profiles, load patterns, etc.