Qvoppies Free Social Icons
Free 48 icons + 10 alternative icons + PSD
Qvoppies Free Social Icons is a free Iconset design resource made by Dribbble user Ampeross. As far as I can tell, its license is Creative Commons Attribution-NonCommercial-Share Alike 3.0.
Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.