How to recognize Facebook User-Agent

"Facebook's user-agent string is facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)..."

Hi

Small, yet important, correction -> Facebook external hit uses 2 different user agents:

facebookexternalhit/1.0 (+http://www.facebook.com/externalhit_uatext.php)
facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php) 

Setting you fitler to 1.1 only may cause filtering issues with 1.0 version.

For more information about Facebook Bot (and other bots) please refer to Botopedia.org - a Comunity-Sourced bot directory, powered by Incapsula.

Besides user-agent data, the directory also offers an IP verification option, allowing you to cross-verify an IP/User-Agent, thus helping to prevent impersonation attempts.

For list of user-agent strings, look up here. The most used, as of September 2015, are facebookexternalhit/* and Facebot. As you haven't stated what language you're trying to recognize the user-agent in, I can't tell you more information. If you do want to recognize Facebook bot in PHP, use

if (
    strpos($_SERVER["HTTP_USER_AGENT"], "facebookexternalhit/") !== false ||          
    strpos($_SERVER["HTTP_USER_AGENT"], "Facebot") !== false
) {
    // it is probably Facebook's bot
}
else {
    // that is not Facebook
}

UPDATE: Facebook has added Facebot to list of their possible user-agent strings, so I've updated my code to reflect the change. Also, code is now more predictible to possible future changes.