To globally clear out Facebook pre-filled messages, you will need to edit the appropriate entry in the Message Catalog Editor. 
  1. Setup>Product Configuration
  2. Under the Message Catalog List, search for “View my Page”
  3. Click Edit Custom next to View my page (trpc:class_social_share_dialog_message)
  4. Enter a space into the Custom Text field box
  5. Click Submit 
Please note that you must be an Advanced Site Administrator in order to make this change.  If you do not have a Product Configuration option under the Setup menu, contact support to make this change for you.