Dogs frequently rub their muzzles and faces on the carpet or grass as a way of scratching them when they itch or to help dry them or get rid of excess drool. In many cases, we see this in dogs who have an inhaled allergy condition or skin dermatitis.
It might just be a little skin irritation possibly related to an allergy condition. Many dogs do this when their eyes itch as well, so check his eyes for any redness as well.
Itchy eyes and muzzle can indicate an inhaled allergy especially if you are seeing some sneezing as well. You can read about inhaled allergies here: http://www.sniksnak.com/doghealth/inhalants.html
If this is the case, Benadryl can be given to your dog, the dose is up to 2mg per pound every 8 hours. Though we usually start with 1mg per pound to start with since you want to use the smallest dose that is effective.
Additionally, dogs do have some scent glands on the face and will rub the face against objects to transfer their scent to them. However, it seems your dog is doing this excessively and thus it is more likely to be allergy related. There is sometimes a dermatitis associated with allergies such as staph or yeast dermatitis. Staph usually occurs on the lower regions of your pet and tends to have small pimple type bumps. Shampoo containing Chlorhexiderm and/or Oatmeal can help with this condition though it does not cure the allergy. Yeast typically shows as a greasy area that has a sweet musty odor. Sometimes the skin can become inflamed, darker and thickened due to itching. Yeast likes areas such as between toes, armpits and ears and moist areas which can include around the mouth on the muzzle. Selsun Blue Shampoo can help with Yeast dermatitis. Miconazole cream can also be used as a topical cream for yeast as well. When shampooing, lather and leave on 15 minutes before rinsing. Your Vet though will make the final determination.
You can try the benadryl and miconazole cream and see if there is any improvement. If not, then you should have your vet examine the area. I hope this information is helpful to you.