You know the concept of “peak oil,” yeah? That we will at some point reach the highest level we’ll be able to draw from the ground, and it’s all downhill from there.

Facebook’s way slower growth last month has people contemplating whether Facebook is peaking, at least in the US. The blog Inside Facebook says FB gained only 330,000 US users in June. Sounds like a lot — that’s approaching the population of Minneapolis, as I recall.

But consider that one month before, in May, they logged 7.8 million new users. That’s the kind of growth the site had been accustomed to.

Of course, FB faced user backlash and harsh media attention right around this time for perhaps overplaying its hand when it came to sharing and exposing user data. If this is largely due to that dust-up, then it’s a HUGE effect (much larger than Facebook let on when they said the controversy had no discernable impact on their numbers). But even if millions were turned off by the negative attention, it’s a temporary effect.

On the other hand, when I reported on that episode for Marketplace, it was not hard to find users already experiencing Facebook Fatigue, questioning why they had invested so much in it anyway. This would seem to be the much more intractable problem for Facebook, if users are simply getting their fill. Once people start to bail on a social network, it can be hard to reverse the decline. (Ahem: MYSPACE. Gesundheit.)

Facebook’s brightest spot is overseas, where it seems set to match (Google’s) Orkut in India and Brazil. Global domination may yet come before the peak.