There are others here who are much better at Smarty than I am, but I don't think I'd be wrong in pointing you to "entries.tpl" in whatever theme subdir corresponds to the theme that you're using (/templates/i3theme/entries.tpl in my case).
I expect for most themes that there is a block above {$entry.add_footer} which contains the author tags ($entry.author) where you could put your FB stuff.
What I'm not sure of is how the FB stuff is supposed to act...is it supposed to fetch the blog URI as part of some sort of FB post?
Anyway, I hope this at least gets you pointed in the right direction.
Adam
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Ah - I see (got curious because I'm becoming a FB addict it seems). The block of script you provided basically does all the work for you - fetches the background image to make the href look like a button, sends the current url to FB which prompts for login unless you're session is already active, etc.
Pretty cool - maybe I'll add this to my my site but stuff it in the {if $entry.is_entry_owner and not $is_preview} ... {/if} condition so that only I can push my blog entries to FB...but that's just personal preference.
Do you mean the "Findmore" plugin? You would need to edit the plugin to create this new option.
Quite frankly, I think it is generally recommended that users add the component pieces they need from that plugin to their entries.tpl file. You gain a small performance improvement in the process.