More than half of the 25 largest games on Facebook lost users in April, but the trend picked up last month.

Is the Facebook platform collapsing? That’s not what the data is showing. Social game developers of all sizes continue to see growth (check out our weekly top 20 lists for more on that).

Why, then, are so many of the largest games losing so many users? One reason could just be age. Almost all of these apps were launched last year. Game lifespan varies depending on genre, quality, the number of updates, and other factors — and some of these titles, like Restaurant City and Farm Town, are more than a year old.

But another factor is obviously Facebook’s removal of notifications at the beginning of March. That channel was a key way that most games grew and maintained user engagement last year. Facebook removed the channel because so many developers used it so aggressively that it became filled with spam. With each change in user communication, developers have to adapt their game design – but it’s often hard to prevent at least some period of traffic loss.

What’s left for developers? Organic growth through news feeds, email and the soon-to-change requests features — and advertising and cross-promotion.

These channels provide slower growth, but are also harder to spam; send too many emails to a user about a game and they’ll filter out the sender.

Facebook’s platform has gone through jarring realignments in the past, and developers are once again searching for new ways to build their businesses. We expect they’ll optimize successfully now, despite the losses.

However, one jarring note is that Playfish’s Hotel City, Playdom’s Social City and Zynga’s Treasure Island — three new hits from the three largest social game developers on the platform — are also registering declines. Because these large developers depend heavily on internal cross-promotion, network-wide traffic declines are also impacting their new app growth. This is also likely driving up the cost of paid acquisition, meaning we could be seeing decreases in the cost-effectiveness of that channel as well.

Here are the highlights for the Top 25 Facebook games for June, 2010:

• Zynga‘s FarmVille sees the biggest drop yet, losing upwards of 7.7 million monthly active users. With the tremendous amount of marketing being done by the company, however, like partners such as 7-Eleven, it will be curious to see how things go next month.
• Texas HoldEm Poker moves up to #2, to pass RockYou‘s Birthday Cards. The loss for the former was around 740,000 MAUs compared to the latter’s, 6.3 million.
• Newer title Treasure Isle gives us a clear idea of how much Facebook’s third-party notification removal has affected player returns. While the game has moved up from #5 to #4, it lost roughly half a million users over the last month.
• Long running meta-game MindJolt Games jumps back up to a more common local at #8 (up from #13), with the only notable gain this month of over 2.8 million MAUs. Perhaps MindJolt is on to something with meta-titles, but then again, it’s always fluctuated in numbers.
• Playfish’s (Electronic Arts) Pet Society (16 million MAUs) moves back up to #9 to pass up the two fishy titles, CrowdStar‘s Happy Aquarium (15.5 million MAUs) and Zynga’s FishVille (14.9 million MAUs).
• Hotel City, from Playfish, and Social City, from Playdom, make another possible example of the third-party notification removal, as these two newcomers from last month see a drop in users at roughly 1.4 and 1.1 million respectively.
• Of all the losses, PopCap’s casual king, Bejeweled Blitz sees the smallest, coming in at #17 with a 232,146 MAU decline.
• Farming gives way to happiness as Country Life and Farm Town are beat out by CrowdStar’s Happy Island and Happy Pets, who come in at numbers #18 and #19.
• And last, but not least, the ninja role-playing game, Ninja Saga makes a reappearance on the charts at #25 with 5,351,082 monthly active users.