Since July 17 the old share button stopped working on all the websites I manage.
I've read reports by other users that are facing similar issues, but I was not able to find anything official.
Do you know if there is any official source available for this?
Update:
As of May 2020, the Facebook documentation doesn't explicitly mention support for /sharer.php URLs, but the snippets generated by Share Button page still refer to that path.
Some time has past since this first was asked but some new information from the Facebook documentation has given us the possibility to see some official "proof" that the sharer.php method of sharing links still works, is still supported and is alive and kicking.
The reference is this page of the documentation and a few examples are also given:
This is what the simplest use of the share dialog looks like:
<a href="https://www.facebook.com/sharer/sharer.php?u=example.org" target="_blank"> Share on Facebook </a>
And also:
You can point to this URL in a new window or a popup. If you use JavaScript to pop open a new window, you can also refer to the URL of the current page being shared as location.href. That gives you code you can use on all pages:
var sharer = "https://www.facebook.com/sharer/sharer.php?u="; window.open(sharer + location.href, 'sharer', 'width=626,height=436');
To summarize, there was indeed a period of time when the behavior of the sharer.php was unpredictable. It was widely accepted that the method had been deprecated and that it was not wise to use it. This has now officially been busted as an example usage of sharer.php was added to Facebook's documentation.