How to recognize Facebook User-Agent

When sharing one of my pages on FB, I want to display something different. Problem is, I prefer not to use the og: elements, but to recognize FB user-agent.

What is it? I can't find it.

Solution

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.