Just how many people in Tonga are LDS depends on how you count them. According to Wikipedia:
"The current membership of the church is 60,680, which represents 57% of the population of Tonga. As of year-end 2007, the LDS Church reported 54,281 members, 16 stakes, two districts, 125 wards, 39 branches, one mission, and one temple in Tonga. The claimed membership total represents approximately 46 percent of the population of the kingdom.
In 1996, the LDS Church reported about a third of the Tonga's population to be members. However, according to 1996 census data, 14 percent of the population self-identified as Latter-day Saints at the time. LDS Church membership statistics are different from self-reported statistics mainly because the LDS Church does not remove an individual’s name from its membership rolls based on inactivity in the church."However, another website reports "Today more than 90 percent of the Tongan population practices some form of Christianity as a result of missionary settlements from the Free Wesleyan Church, the Roman Catholic Church and the Mormon Church over the last two centuries. Christian churches are abundant on Tonga, an island in the South Pacific, but traces of indigenous religious and cultural beliefs still remain, evident in taboos and beliefs rooted in spirituality and animism."
Vava’u has the largest concentration of Mormons (18.5%) whilst Niuafo’ou and Niuatoputapu have the lowest (only 4% or 59 people). The village with the largest Mormon concentration was the village of Matahau in Tongatapu with 400 of its 628 residents identifying as Mormon. The church is also predominant among the Tongan diaspora in the United States.
The Church makes a deliberate effort to be good neighbors and endeavors to get along as best as they can with the other faiths. Here are some of the churches we saw while we were on Vava'u.
For more info on other topics about Tonga see Encyclopedia.Com