This was not working for me and I was getting very frustrated as all old posts did not have the Alert Box buttons displaying. I had no idea where this stuff was even saved. So I dived into the code.
I figured out it was a bug in the socialize-frontend.php file.
Instead of using the default boxes in the alert box it just relied on the post having them set custom for each post. That's why they were showing up on new posts but not on all the old ones.
To fix the issue, simply replace the following lines in the "socialize-frontend.php" file located in the "frontend" folder of the Socialize plugin.
Replace this at line 161:
$socializemeta = explode( ',', get_post_meta($post->ID,'socialize',true));
With these lines:
$post_socialize_meta = get_post_meta($post->ID,'socialize',true);
$sm_str = !empty($post_socialize_meta) ? $post_socialize_meta : $socialize_settings['sharemeta'];
$socializemeta = explode(',', $sm_str);
Used this fix on CampTrip and it's all working fine now.
I hope this helps some people. Happy blogging :)