social theme and browsers support

Recently some of the visitors on my site that uses Social Theme complained that they cannot see the page (only blank page). It turns out that all of them were using browser Internet Explorer v7.
Could you please let me know what is the oldest IE browser that should still work with Social Theme?
The site is available at http://svn.amebylon.net/ and uses all the latest software versions: WP 3.3.1, BP 1.5.4 and Social 1.3.4.