Yes, you can. Just remember these three rules:
All icons up to 100*100 px are available for free in the PNG format, but link attribution is required.
Icons from the Popular category are available for free in all formats and sizes, but again, link attribution is required.
Icons from the Logos and Characters categories are available for free in all formats and sizes, link attribution is required. All matters relating to the commercial use of such icons should be settled with the respective owners of logos/characters.