facebook social plug-in not showing up when added dynamically

I am adding facebook social plug in to a webpage

when I manually add :

<div class="fb-comments" data-href="http://website.com/z" data-width="700" data-numposts="7" data-colorscheme="light"></div>

it works , however , when javascript code add it , it doesn't

any ideas ?

The JS SDK goes through your document once when it is initialized, to look for such elements to parse into social plugins. If you want it to also parse content that you add to the document later, you need to call FB.XFBML.parse().