I think it should load at the bottom just fine, but you'd need to test and make sure of that for your configuration. In general, it *is" load-blocking JS but most browsers nowadays will do look-ahead loading and continue on anyways... so the impact is usually small to non-existent.
Those messages are quite annoying though, so I understand why you'd want to fix it.
Finally, if you're still using Joomla 1.5, please take the time to read our
Facebook Graph API v2: What's Changed and What it Means
blog post. Specifically, the "Ugly" section about Joomla 1.5 compatibility with the recent updates to Facebook.
I hope that helps, and best of luck,
Alex