You can add a standard share button on your canvas app or custom fan page in Facebook but using the code supplied on their documentation. The problem is it behaves differently to like/share buttons on an external website. Simply adding the usual meta data and code below

<fb:share-button class="url" href="http://www.example.com/" />

You are left with a rather bland looking wall post or message. 

What Facebook actually offers here is a nice way to modify the message, and image, of each share icon on a single page. this is done by adding the share meta tags within the fb:share-button tag, so the following code:

<fb:share-button class="meta">

<meta name="medium" content="mult"/>

    <meta name="title" content="Example site" />

    <meta name="description" content="Check out example.com" />

    <link rel="image_src" href="http://upload.wikimedia.org/wikipedia/commons/a/a9/Example.jpg" / >

    <link rel="target_url" href="http://www.example.com/"/>

</fb:share-button>

Gives this as a post:

 

Which is a great, but slightly hidden, feature of FBML