Looking at your site, the header is still only set to a z-index of '10'. The line I'm looking at is below:
@media (min-device-width: 1025px) and (min-width: 960px)
#block-header {
z-index: 10;
}
The file is gzipped and compressed. If you updated that z-index to 9999, it should work, but you may just need to clean your Joomla cache to reload that file.
Let us know what you updated, if it wasn't the line above, but with that change, the header should definitely stay on top of our social buttons.
Thanks,
Alex