I tried various codes and none worked. This is one of those:
<?php
$title=urlencode('Title of Your iFrame Tab');
$url=urlencode('http://www.example.com/');
$summary=urlencode('Custom message that summarizes what your tab is about, or just a simple message to tell people to check out your tab.');
$image=urlencode('http://www.daddydesign.com/wp-content/uploads/2011/06/facebook_iframe_share_button.jpg');
?>
<a onClick="window.open('http://www.facebook.com/sharer.php?s=100&p[title]=<?php echo $title;?>&p[summary]=<?php echo $summary;?>&p[url]=<?php echo $url; ?>&p[images][0]=<?php echo $image;?>','sharer','toolbar=0,status=0,width=548,height=325');" href="javascript: void(0)">Insert text or an image here.</a>
Facebook have changed something and now the only parameter that a developer can change is the URL of the page, but nothing else.
Does anyone know if it's still possible to change the title and description of the shared page?
Consider using Facebook Open Graph.
The Open Graph Protocol website.
Also there is a plugin for Open Graph.
An example:
You can change the title and description which appears in shares with these meta tags.
<meta property="og:description" content="Your description here" />
<meta property="og:title" content="The title of the page" />
Facebook also provides a debugging tool which you can use to see how it sees a given page from your website.
Sometimes Facebook seems not to clear the cache for a given page. There's a discussion about that issue here on SO. I have trouble with that sometimes. For me it works puting something like ?refresh=1 at the end of the URL and then increment that number each time.
You can see a list of best practices on the use of Facebook OG here. Last but not least here is a list with more available tags. You can click on each one to see more info. I fear it is not comprehensive, though as it does not include for example og:title.