Just stumbled across this website with a huge selection of freely available icons. Your basic share icons or payment method icons are there but also a wide selection of user made additions.
SoftIcons.com allows to download icon sets free of charge (at least for personal use). The catalog contains 475,655 icons/images in 2,849 sets. Popular tags: facebook, twitter, home, apple, search, photoshop, phone, folder, cloud, arrow, book, youtube, linkedin, pinterest.