I think you mean that you want the Social Bookmarking plugin to appear above iPaper, right? I don't see the MyOmeka plugin.
Both of those plugins tap into the same hook in the public theme, so you're right that they're currently ordered based upon when they were installed. There's currently no user interface for changing this order. However, there's an easy fix if you edit your plugin's code.
Open up the social bookmarking plugin, and comment out line 9. It currently looks like this:
add_plugin_hook('public_append_to_items_show', 'social_bookmarking_append_to_item');
so make it look like this:
//add_plugin_hook('public_append_to_items_show', 'social_bookmarking_append_to_item');
This basically prevents the plugin from automatically inserting the social bookmarking links into your theme. You can not re-insert them into your theme by pasting the function social_bookmarking_append_to_item(); anywhere in your theme's code.