What PHP Framework does Facebook use ? Can it be downloaded from anywhere ???
You can download the Facebook SDK from http://developer.facebook.com, it contains all the dependency within it, and can be used 'as is' without additional downloads.
If you are trying to create something similar to Facebook, you could also try Elgg. http://www.elgg.org/ – Open Source social network engine. It creates a mini-Facebook where you can friend each other, send private messages, have discussion forums, host your own person blog, etc.
Funny you should mention it, I was just watching a presentation on how facebook is put together. You will see from it that they partially use PHP, but it is in a customized form.
Also find their use of Memcache very intriguing. They've used it with a home baked UDP mechanism to give them the scalability that they need. It's overall simplicity of solution is worth beholding.
What PHP Framework does Facebook use ? Can it be downloaded from anywhere ???
No idea what framework they use, it'll be a custom solution they developed in house. It's not going to available anywhere to download.
Check out this framework question if you're looking for recommendations on which framework to use.
What PHP framework would you choose for a new application and why?