Can you batch rename files if they contain certain criteria?
From the Finder, select all the files you want to rename, and under the file menu select rename. This brings up a pane with options to rename. For your files, you can put .srt in the find box and replace with _bg.srt. Then you can replace 'File1' with 'Facebook'. This will quickly rename the files for you. Looking around the rename panel, under the first pulldown on the left is the option to reformat. This option may come in handy as well.