The exponential growth of the SGI stopped no later than 1972

The numbers cited here (until the map, below) are all taken from SPIRITUAL INDEPENDENCE: An Introduction to Soka Spirit

Summary - the numbers:

1972: 8 million members

1991: 10 million.

1991 or 1997: 10 million.

In 19 years (or possibly 25 years), only 2 million members were added worldwide. To add 2 million to 8 million, that means that, over those 19 years or 25 years - an entire generation! - only 1 out of 4 members was able to shakubuku a SINGLE person. And there's no guarantee that person continued practicing after receiving his gohonzon!

2008: 12 million

Again, in either 11 years or 17 years, that's another 2 million worldwide. Now, within that time frame, we're looking at 10 million people only converting 2 million people - or one out of five members managing to talk a single person into receiving gohonzon that entire time! Again, no information on retention rates.

The number seems to be holding steady at 12 million.

They now claim 12 million adherents, worldwide, but most consider this number a great exaggeration. Source

What can we conclude from this?

The SGI wasn't experiencing exponential growth since 1972, despite adding so many foreign offices. Here is the current information:

12 million members of Soka Gakkai International (SGI) around the world - from the SGI's home page

No increase at all over the past 6 years - and that's including every person who ever joined, only (maybe) subtracting the ones who officially resigned via a letter of resignation submitted to the LA head office! That's maybe 20 people tops O_O I'm one of them :D

For purposes of comparison, the world population in 2008 was estimated at 6.1 billion. The world population for 2013 is estimated at almost 7.2 BILLION. The population is growing; SGI's membership is stuck at the 12 million point - or even less!

Here is a map that shows SGI membership by continent: SGI's own map

It's a very poorly put-together source, because some of the figures are households, while others are individual members. But here are the numbers (with commentary on each continent/area):

North America: 352,000

This number is certainly false.

You may know that NSA issued over 800,000 Gohonzons from 1960 until 1990. ... By the beginning of 2004 our total membership nationwide was roughly 70,000. - Danny Nagashima

70,000 out of 800,000 = 0.0875, or less than 10%. That's a shocking admission, and it's a scandalous defection rate.

In the 1980's, the current SGI-USA General Director Emeritus George Williams claimed a membership of 500,000 and a World Tribune subscription base of 100,000. However, it is a certainty that today in 1994, there are 20,000 World Tribune subscriptions. This is a surprising decrease.

Furthermore, Vice-General Director McCloskey tells the mass media that the SGI-USA has 350,000 believers, but recently, he admitted to a certain group of people that the actual number of members is close to 20,000, the same number as World Tribune subscriptions." Source

SGI announced a goal at the beginning of 2014 of raising World Tribune subscriptions from 35,000 to 50,000 Source

So there's our number: 35,000. 1/10th of the 352,000 claimed by the SGI.

Central America: 20,000

South America: 236,000

Brazil is the largest SGI location, because Brazil has the largest Japanese ex-pat population in the world. It's a no-brainer. I don't have a more accurate or independently validated number.

Europe: 105,000

Middle East and Africa: 25,000

Asia and Oceania: 1,017,000

Japan:8.27 million households

Out of the 10 million Soka Gakkai members, 2.5 million regularly participate in religious meetings and try to increase membership, according to Hiromi Shimada, a religion scholar who has written several books about the group.

Soka Gakkai has 1.69 million members outside Japan. Source

Soka Gakkai International claims a total of over 12 million adherents. The lion's share of these belong to the Japanese organization, whose official membership count is 8.27 million households. This number has been questioned by several authors: Murakami (2007) suggests 2.5 million individuals, Shimada (2007) 2.5 million households, and Numata (1988) around 5 million individuals. According to statistics from the Agency for Cultural Affairs (a body of the Japanese Ministry of Education), the Japanese organization had 5.42 million individual members in 2000. Source

We all know it's the "actives" that are the only numbers that count. In Japan as everywhere else, as in every other religion, they count everybody who's ever signed on as an "active" member, even if they've left and joined a different religion. It's a deceptive and manipulative tactic to appear bigger and stronger than they really are.

There are sources that have claimed that, in the wake of the numerous political scandals that led to the Komeito Party having to redefine itself without any Nichiren aspects, as "New Komeito", its membership stopped growing:

...the relative powerlessness of Komeito after 1970 [when the Komeito was forced to shut down and restructure as New Komeito without any Soka Gakkai elements]... Source

Up to recently, the New Komeito Party's success lay in strategic alliances with more powerful parties and an emphasis on pork:

This complementary relation between the LDP and Komeito actually has a prehistory. On the level of the local government, Komeito joined the ruling coalition since the mid-1970s in one place after another. Here, on the local level, the LDP and Komeito have worked together for more than thirty years in a conservative political frame that involves the inducement of specific profit to localities through political clientelism, including, for example, public construction dependent on governmental subsidies. The result of the 2009 general election, that is, the defeat for both the LDP and Komeito, probably indicates the malfunction of this political style. Source

For more discussion on this topic, see here

We'll count 2.5 million "actives" as the REAL membership in Japan

Conclusion: We'll use the numbers above. Let's count (in thousands):

35 + 20 + 236 + 105 + 25 + 1017 + 2500 = 3938

3,938,000 SGI members worldwide. Not quite 4 million members. A truly miniscule 0.056% of the world's population. About half a tenth of a percent.

There are about that many Jains in the world. Jains! There are also that many Shinto devotees in the world, even though there are basically NO Shinto devotees anywhere except Japan.