That actually looks like an RFI attack being blocked since this is simulating an RFI hacking attempt against your website. You can try whitelisting the l.php filename and/ or maybe the facebook domain.
# TIMTHUMB FORBID RFI and MISC FILE SKIP/BYPASS RULE
# Only Allow Internal File Requests From Your Website
# To Allow Additional Websites Access to a File Use [OR] as shown below.
# RewriteCond %{HTTP_REFERER} ^.*YourWebsite.com.* [OR]
# RewriteCond %{HTTP_REFERER} ^.*AnotherWebsite.com.*
RewriteCond %{QUERY_STRING} ^.*(http|https|ftp)(%3A|:)(%2F|/)(%2F|/)(w){0,3}.?(blogger|picasa|blogspot|tsunami|petapolitik|photobucket|imgur|imageshack|wordpress\.com|img\.youtube|tinypic\.com|upload\.wikimedia|kkc|start-thegame).*$ [NC,OR]
RewriteCond %{THE_REQUEST} ^.*(http|https|ftp)(%3A|:)(%2F|/)(%2F|/)(w){0,3}.?(blogger|picasa|blogspot|tsunami|petapolitik|photobucket|imgur|imageshack|wordpress\.com|img\.youtube|tinypic\.com|upload\.wikimedia|kkc|start-thegame).*$ [NC]
RewriteRule .* index.php [F,L]
RewriteCond %{REQUEST_URI} (l\.php|timthumb\.php|phpthumb\.php|thumb\.php|thumbs\.php) [NC]
RewriteCond %{HTTP_REFERER} ^.*example.com.*
RewriteRule . - [S=1]