Ok i had a on adding some more details on a recent post so i am going to put some more light on the post Facebook Learned SEO posted earlier.
As we talked earlier that facebook have used a rewrite for member profiles. First we will talk a little about rewrites. Rewrites are of different sort but there i mean .htaccess rewrites. Its a process through which a url can be written and shown according to your wish.
For example: if you have a site developed in php and all files have .php as their extensions. for example www.google.com/index.php through your .htaccess you can rewrite it as www.google.com/index.html , means that u can change the .php to .html . Not just extensions but also whole file names, urls etc. can be changed.
Dynamic Urls can be changed to static , a real example:  www.blastingart.com/profile.php?ID=1 . This url shows the profile of user whos id =1 , while his username is Ayaz-Malik. Now by adding some .htaccess rules. it can be rewritten and used as www.blastingart.com/ayaz-malik.
Try it your self:
http://www.blastingart.com/profile.php?ID=1
http://www.blastingart.com/ayaz-malik
both will open the same user profile. Now why should use rewrites like these??
There are a dozen good reasons for that im gona add the top priority ones.

• SEO Friendly Google and other search engines counts urls like profile.php?ID=o2323 or any urls like these as one. Means they will tream profile.php?ID=1 and profile.php?ID=23 as just one file/page which is profile.php, but every url with different username or any other parameter used is different. For Example: www.blastingart.com/ayaz-malik and www.blastingart.com/goku  are two different pages for google and other search engines.
• Every rewrited url can have page rank assigned, while if page rank assigned to a profile.php type of urls will remain same for all
• Easy to remember, read, use Members, visitors can easily be remebered, readable and usable. Most of all they makes sense. A user always gets the point what does that means. Lets take another example. If u have a shopping cart and your selling a mobile phone named as phoneX. The url to your product selling page would be something like this www.yoursite.com/products.php?ID=(what ever the id is). But if u use user friendly permalinks or rewrites. it would be like this www.yoursite.com/products/mobileX

I think thats enough and understandable. So now this is the same feature which facebook always lacked and was behind in its SEO. But now they hae adopted this system and the user profiles are rewritten as www.facebook.com/username. That means every profile will haev its own page rank, will be easily remembered , understandable and most of all will be counted as a seperate page on search engines easily!!