You mentioned that you deleted cookies but try this also...
From your Safari menu bar click Safari > Preferences then select the Privacy tab.
Click: Remove All Website Data
Quit then relaunch Safari, try FB.
If that doesn't help, boot in Safe Mode
A Safe Mode boot takes longer then a normal boot so be patient. This will delete system caches that may help.
Once you see the Desktop, click the Apple menu icon top left in your screen. From the drop down menu click Restart.
Check System Preferences > Network > Advanced > Proxies
If any of the boxes on the left are selected, deselect then click OK.
Quit then relaunch Safari.
And check Safari > Preferences > Extensions. If you have any installed, turn that OFF, try FB.
If nothing above has helped, try deleting the cache associated with Safari.
Go to ~/Library/Caches/com.apple.Safari/Cache.db.
Move the Cache.db file to the Trash.
One other thing to try is to delete your Facebook keychain.
Open Keychain Access located in /Applications/Utilities
Select Passwords on the left.
Type facebook in the search field top right corner of that window then right or control click the Facebook keychain then click Delete.
Back to Safari. From the Safari menu bar click Safari > Preferences > Autofill
Make sure User names and passwords is selected.
Go to Facebook, enter your user name and password. Click Yes when prompted to save that data to a new keychain.
And try using a different browser.Mozilla Firefox Web Browser
edited by: cs
Doesn't help you at the moment, but you aren't alone > http://answers.yahoo.com/question/index?qid=20120517001849AAc3I94
You might try creating a new account > https://www.facebook.com/index.php?stype=lo&lh=Ac9MiEY2u-Q456Np
This looks like a DNS cache issue.
Flush your DNS cache regularly (I set all my Macs to flush daily at boot): http://applehelpwriter.com/2012/04/16/flush-dns-cache-in-lion
Try to use a different DNS server (eg 184.108.40.206) directly from your computer rather than via your router or ISP.
Adjust your Reset Safari settings to something you are happy with and in future start Safari via the following AppleScript. I use this script on old Macs to increase Safari speed. The first line overcomes the reluctance of some Safari versions to remove local storage when resetting. Local storage is sometimes used as an alternative to cookies by HTML 5 pages - Facebook uses HTML 5.
do shell script "rm -Rf ~/Library/Safari/LocalStorage"
tell application "Safari" to activate
tell application "System Events"
tell process "Safari"
click menu item "Reset Safari…" of menu "Safari" of menu bar 1
click button "Reset" of window 1
"Neville Hillyer" That was an excellent answer, and has fixed my problem as well. For googlers, and forum hunters looking for an answer to the dreaded "invalid url "/" " problem, flush the DNS cache is simple (This is for Mountail Lion anyway)
1:) Goto "Finder"
2:) Select "Utilities"
3:) Select "Terminal"
After opening a new terminal session, simply copy the following command line, and paste it into the Terminal session.
sudo killall -HUP mDNSResponder
That will flush your DNS cache (OSX will ask for your admin password, which you will be prompted to enter in Terminal)
Hopefully that will solve a lot of problems, again, thanks to Neville and Phil Stokes.