Thanks for your detective work, offgridpro. I've downloaded AddThis and reproduced the problem.
The issue is that AddThis is doing something funky to the_content, and they're doing it *after* other filters have run. You can temporarily fix the problem by finding the following line in addthis/addthis_social_widget.php (around 1067):
add_filter('the_content', 'addthis_display_social_widget', 15);
and changing the number 15 to something less than 10, eg
add_filter('the_content', 'addthis_display_social_widget', 5);
It's possible that we could do a better job avoiding these kinds of conflict in BuddyPress. I'm moving this ticket to 1.8 so the dev team can discuss.