The most popular type of facial treatment requested by customers at The Estheticians, Inc. spa in San Mateo is the Signature Facial. This treatment includes a deep cleaning, exfoliation, extractions (if necessary), and a customized mask followed by a relaxing massage to soothe the skin. It's perfect for all skin types and can be tailored to address specific concerns such as acne, dryness, or aging. Additionally, The Estheticians offers other specialized facial treatments like Microdermabrasion, Oxygen Facial, and Peels, which are also quite popular among customers.