An excerpt from http://www.sitepoint.com/14-hand-picked-free-social-icon-packs/, by @adaivanoff
Of the thousands of icons you can find online, social network icons would have to be among the most numerous.
This is hardly surprising, considering the amount of time people dedicate to social sites and the rewards they offer when your users share your content to these networks. Because of this, you can only benefit, if you add social icons on your site or on the sites you design for clients.
In the previous article in the series, I focused entirely on flat icons. However I found so many attractive ‘non-flat’ social icons I decided to broaden my criteria with this piece.
There is also no doubt that vector icons are more useful because of all the flexibility they offer – and the rise of SVG adoption – and I set out to avoid raster icons all together. Again, I felt this would have been unfair as it would have excluded some of the finest examples in the category. And raster icons are still widely in use and this will hardly change in the near future.
And now, here is my shortlist of hand-picked free social icons: