Tongue Piercing now a days becomes popular as it also is taken by many world celebrity. But behind tongue piercing a long history has been. Actually tongue piercing isn't only popular at present but also in past time.
The most and informative history about tongue piercing with both Aztec and Maya cultures. In that time tongue piercing was done by priest. They pierce their tongue for either passing rough cards or drawing blood from the hole. But it is sure that no evidence of long term or permanent of tongue piercing in Aztec culture was. Anyway, it is sure that in that time it was done to honor the God.
Nevertheless you can confirm that tongue piercing have a long history in religious and performance practices around the world.
In Maya cultures, like Aztecs they also did it for their God. In middle east some Fakirs or Sufis did tongue piercing as an offering and proof of trance state. Also Some Asian spirit Mediums of the Far East also did it with same interest like Fakirs or Sufis.
In Australia tongue piercing was practiced by the central Aboriginal Australian holy man who used it to suck with his tongue the magic out of his patient's body.
From the beginning of 20th century, tongue piercing becomes very popular over the world specially in U.S.A. and Europe. To familiar with tongue piercing Western Carnies borrowed many sideshow tricks from fakirs to American and European audience. After that it was getting a common to the western people.