Many of the largest, earliest-established countries on Facebook have seen odd traffic fluctuations in the last months, according to our Inside Facebook Gold data. Canada, for example, has gained or lost tens of thousands of users since the middle of last year, and in April lost near half a million users.

But what’s the overall trend? We took a closer look at our historical data to see if larger trends emerge. We graphed the growth rate each month going back to the start of 2008 for five of the first countries to see very broad adoption of Facebook, also including the United States, the United Kingdom, Turkey and Australia.

The results: You can clearly see the overall decline over time, with each of the five falling towards zero. Obviously, percentage gain here can be deceiving because any growth is a lot, proportionally, at the start, but less so as a given country gains. So you’d expect what you see here — sort of. It’s actually surprising to see how many countries maintained growth rates of 5% or more through the middle of last year.

But even if early-adopting countries started losing users, you’d still expect it to not fall below zero given how sticky the site has been with users of varying age, gender, and regional groups. Growth might fall to zero, but why would it go below if half of Facebook users come back every day, as Facebook says?

Except for Turkey, which only has around a third of the country on Facebook, the rest of the countries have roughly half of their populations on Facebook. We’ve previously observed that this is around where a country of any size will stall. The graph here provides further support to that observation.

In these countries, it does appear that recently fewer people come back than had in previous months, without new users making up for the losses. The severity and frequency of the drops appears to have increased starting last July, and the recovery gains have been weaker than in past years had been after drops.

The average decline over months has yet to become negative, though. There are other possible factors at work, too, such as bugs in the Facebook ad tool that we get this data from.

For anyone interested in Facebook’s future, whether users, developers, marketers, investors or others, identifying the long-term directions here is crucial for planning how to capitalize on Facebook’s success, or lack thereof. We’ll continue tracking the trends here and at our Inside Facebook Gold Stats Page.