As I recall (it’s been a while since I looked into it) Facebook only allowed a certain number of characters in a post to the wall of a group.
Looks like that number has been expanding over time, though.
Take a look at lines 317 through 373 of includes/publish_to_facebook.php for where the actual posting occurs, or lines 144-158 for where the “$wpbook_description” gets set.
If you have an excerpt, it uses that; if not, it pulls from the content but truncates at 995 characters to stay within the original 1000 limit.
Add:
$wpbook_description = stripslashes(wp_filter_nohtml_kses(apply_filters('the_content',$my_post->post_content)));
after line 158 to get the full long description back (regardless of whether you put in an excerpt or not).
