As noted above, the easy-tags aren't being rendered because the AJAX isn't calling them. We'll have to try to work through this in a few steps. The first is to use the following raw HTML instead of the tag in the template:
<div class="fb-share-button" data-href="<?php echo $this->event->getLink(); ?>" data-layout="button_count"></div>
My guess is that this will insert the proper HTML into your page, but the button will not display (nothing will). In that case, another small amount of Javascript will need to be executed each time 'new' activity is shown:
The problem is that I don't know where to put that since it will have to execute after a JomSocial call. You may need to contact the JomSocial developers to determine how to hook into their Ajax request the best to add that code after the new HTML (the activity + share button) is inserted into the page.
I hope that helps explain, but if you have any other questions, just let me know.
Thanks,
Alex