SASS Hash as workaround for not being able to interpolate variable names.

After

a {
    display: block;
    width: 60px;
    height: 60px;
    line-height: 70px;
    text-align: center;
    font-size: 30px;
    color: #fff;
    @include transition(background-color 0.3s);

    $media-hash: instagram $instagram-color, pinterest $pinterest-color, google-plus $google-plus-color, tumblr $tumblr-color, feedly $feedly-color;

    @each $media in $media-hash {

      $color: nth($media, 2);

      &.#{nth($media, 1)} {
        background-color: lighten($color ,20%);

        &:hover,
        &:focus {
          background-color: $color;
        }
      }
    }

    &.feedly {
      background: lighten($feedly-color,20%) image-url('sidebar/sprite-connect.png') no-repeat 15px 17px;
    }
  }//a

Before

  a {
    display: block;
    width: 60px;
    height: 60px;
    line-height: 70px;
    text-align: center;
    font-size: 30px;
    color: #fff;
    @include transition(background-color 0.3s);

    &.instagram {
      background-color: lighten($instagram-color,20%);

      &:hover,
      &:focus {
        background-color: $instagram-color;
      }
    }//&.instagram

    &.pinterest {
      background-color: lighten($pinterest-color,20%);

      &:hover,
      &:focus {
        background-color: $pinterest-color;
      }
    }//&.pinterest

    &.google-plus {
      background-color: lighten($google-plus-color,20%);

      &:hover,
      &:focus {
        background-color: $google-plus-color;
      }
    }//&.google-plus

    &.tumblr {
      background-color: lighten($tumblr-color,20%);

      &:hover,
      &:focus {
        background-color: $tumblr-color;
      }
    }//&.tumblr

    &.feedly {
      background: lighten($feedly-color,20%) image-url('sidebar/sprite-connect.png') no-repeat 15px 17px;

      &:hover,
      &:focus {
        background-color: $feedly-color;
      }
    }//&.feedly
  }//a