UPDATE #3 (June 2016):
It's gone again: There are no more references to the sharer.php in the documentation. Though you have the option to use the share dialog without integrating with the facebook sdk in any form. An important difference is the required app_id
parameter. Here is one example from the docs:
<a id="facebook" href="https://www.facebook.com/dialog/share?app_id=145634995501895&display=popup&href=https%3A%2F%2Fdevelopers.facebook.com%2Fdocs%2F&redirect_uri=https%3A%2F%2Fdevelopers.facebook.com%2Ftools%2Fexplorer" target="_blank">share me</a>
UPDATE #2:
See @Lix's answer.
UPDATE #1:
As Sean pointed out (for this credit to him) you can use the FBML version of the share button. A code snippet for the FBML version:
<!DOCTYPE html>
<html xmlns:fb="http://ogp.me/ns/fb#" lang="en" style="overflow:hidden;">
<head prefix="og: http://ogp.me/ns# fb: http://ogp.me/ns/fb#">
<meta charset="utf-8">
<title>Test</title>
</head>
<body>
<div id="fb-root"></div>
<script>(function(d, s, id) {
var js, fjs = d.getElementsByTagName(s)[0];
if (d.getElementById(id)) return;
js = d.createElement(s); js.id = id;
js.src = "//connect.facebook.net/en_US/all.js#xfbml=1&appId=YOUR_APP_ID"; // appId must be valid
fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));</script>
<fb:share-button type="button_count" href="http://www.foobar.com">
</fb:share-button>
</body>
</html>
NOTE: This version of the share button (like all current social plugins by Facebook) creates a cookie with the datr
parameter which is a potential threat to user privacy. You should keep this in mind if you must follow privacy policies.
A code snippet for the sharer.php version:
<a id="fb-share" style='text-decoration:none;' type="icon_link" onClick="window.open('http://www.facebook.com/sharer.php?s=100&p[title]=foo&p[summary]=bar&p[url]=https://www.foobar.com/&p[images][0]=https://www.foobar.com/thumb.gif','sharer','toolbar=0,status=0,width=580,height=325');" href="javascript: void(0)">
<img src="img/share.gif" width="62" height="18" alt="Share"/>
</a>
ORIGINAL ANSWER:
The Share Button has been deprecated already a while ago. It might be dead completely now, or just temporarily, only Facebook knows. Officially it is deprecated.
From the documentation:
What happened to the old Share button?
We deprecated the Share Button when we launched the Like button [...]
href="https://facebook.com/sharer.php"
Please check this one too. – Fastigium