Unfortunately iOS doesn’t support SVG icons at the moment. And it will probably not have support soon. Also android doesn’t support it.
Update:
It looks like Safari is starting to support SVG favicons
See:
http://caniuse.com/#feat=link-icon-svg
To add a svg icon for pinned tabs within Safari you need to add:
<link rel="mask-icon" href="https://stackoverflow.com/questions/15710047/website_icon.svg" color="red">