Picons Social v.2 Free
Picons Social v.2 Free is a free Icon design resource made by Dribbble user Morphix Studio. As far as I can tell, its license is Unknown.
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.