Yes, it appears that 100% is no longer a valid value to pass to the widget. If you check out the plugin generator at
developers.facebook.com/docs/plugins/page-plugin/, the same behavior is shown.
From this Page Plugin documentation, I would recommend that you could set the width to the maximum of 500px and enable the "Adapt to plugin container width" setting. On load, the page plugin will then adapt down its parent container's width.
The plugin will by default adapt to the width of its parent container on page load (min. 180px / max. 500px), useful for changing layout
If the width of the parent element is bigger than the Page plugin's width it will maintain the value defined in data-width
The plugin will never be smaller than 180px
So there will no going larger than 500px (by Facebook's design) and it will do it's best to resize down for a smaller parent. I think Facebook has just limited what's available for this plugin now.
-Melissa