The W3C validation shows over 12 [Invalid] Markup errors when validating a page using the most recent version of the "social-media-widget" WordPress plug-in.
The errors should be simple enough to fix, as I have fixed them on my site already.
In your social social-widget.php file, below the area you commented as:
/* Display linked images to profiles from widget settings if one was input. */
You many lines of PHP code where a space was not left in front of the "ALT" or "TITLE" tags.
While this doesn't seem to affect the rendering result of your plug-in to the web browser in Mozilla Firefox, it could in other browsers.
May want to fix that!