Image Source: comScore.

When it comes to the important U.S. smartphone industry, there are numerous surveys to gauge market share. Many of these -- Kantar Worldpanel, IDC, and Strategy Analytics, among others -- focus on market share using sales during a given time frame, whether monthly or quarterly. ComScore, on the other hand, centers on measuring market share in terms of the installed U.S. smartphone user base, which includes both new sales, and prior sales minus smartphone retirements.

Unlike the other surveys with seasonal variability -- Apple (NASDAQ:AAPL) gets a large push in the final quarter when the newest and greatest iPhones are released, where Samsung (NASDAQOTH:SSNLF) jumps in the second quarter with the introduction of its Galaxy line -- comScore's data is less variable, and a better reflection of the overall market as it stands now versus the other surveys that are potentially more responsive and predictive of detecting future trends. In addition, and to further smooth its data, comScore uses a three-month rolling average.

More recently, comScore's data has hinted at what Apple CEO Tim Cook has referred to as "Android switchers," a group of new iPhone users who were previously among Alphabet's (nee Google's) host of vendors. While not addressed in depth by Cook, this was widely considered to primarily hurt high-end Android vendor Samsung. Oddly enough, Samsung's largest smartphone threat may not be Apple going forward.

The trend continues
If you were looking for Samsung to reverse this trend, you'd be mostly wrong. According to comScore's August report, Samsung's market share dropped on both a quarter-on-quarter basis and a year-on-year basis. For a visual representation, see the chart below that outlines the top five U.S. vendors by market share during this time frame:

VendorAug '14May '15Aug '15
Apple 42% 43.5% 44.1%
Samsung 28.9% 28.7% 27.4%
LG 6.7% 8.2% 9.1%
Motorola 5.4% 4.9% 4.7%
HTC 4.5% 3.5% 3.4%

Source: comScore.

Not only has Samsung's market share decreased, but this quarter, those losses really accelerated by going from 28.7% in May's report to 27.4% in this one -- a 1.3 percentage-point drop. Remember, this is not a standard survey of sales during a given time frame like other surveys; this signifies fewer Samsung phone users overall, as there are more retirements than those buying Samsung phones.

Two big winners are Apple and, shockingly, LG
Among the top five vendors, there are two big winners from Samsung's loss of market share. The first was Apple, growing from 42% to 44.1% in the last year, according to comScore's data. During the last year, the iPhone 6 and iPhone 6 Plus have proven to be a hit, and many analysts expect the newest iterations to continue to find a receptive audience in the U.S.

However, Samsung may be facing competition from within its ecosystem. During  the last year, non-U.S. traded LG has experienced the largest gain in total installed base market share by reporting gains of 2.4 percentage points. In the last quarter alone, from May to August, LG reported a gain of 0.9 percentage points as its G4 appears to have found a receptive audience in the United States -- enough so to grow its installed base.

During the last half-decade or so, Samsung became the de facto high-end vendor for Android, and picked up market share from users not interested in Apple's closed-walled ecosystem. While many -- myself included -- have always considered Apple its biggest competitor, perhaps Samsung's biggest risk in the U.S. is losing its dominant position among high-end Android users.

Jamal Carnette owns shares of Apple. The Motley Fool owns shares of and recommends Alphabet (A shares), Alphabet (C shares), and Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.