Six of the top ten applications on Facebook are games as measured by Daily Active Users – here are the top six games and their daily active users from DeveloperAnaltics as of June 4th:

But as with any game, a developer has to ask, do they have legs? Will there be a sustainable audience that makes it worth developing a sequel (if you are in the casual download game business) or extending the application to other platforms (as most start out on Facebook and then move on to MySpace, Bebo and Hi5). While daily active users and of course revenue factor into those decisions, what role does user engagement outside the game play in helping make that determination? Facebook Lexicon (below) gives us some indication of how the games are trending, looking at the number of mentions of key phrases on user walls and status updates:

What, no Texas Hold’em Poker? Well Facebook Lexicon only allows you to use two word phrases and there is not a way to filter out generic mentions versus game-specific mentions, so we’ll have to pass on them for this analysis.

The other thing about the Facebook Lexicon tool is that there is no absolute number of mentions, only a relative graph that shows the overall trend. To solve for that, I’m introducing the Facebook Lexicon Activity Pixel Index ™ (or FLAPI) that measures the number of pixels from the x-axis and to give you a number you can use to benchmark competing brands when using Facebook Lexicon graphs. You can then figure out the FLAPI per user to see how engaged individual users are with a specific application.

Here’s an updated chart with the FLAPI and FLAPI per million daily active users:

Note that I took the pixel distance using the LAST date where Facebook Lexicon presented data, which was May 31st. The resulting data looks a lot more interesting when you chart it like this:

Now you can see that when you take the FLAPI and adjust it for the audience, you get a great feel for which brands , relative to their audience, have users talking about their game in their Facebook status and wall pages. In the example above, if you took a 45 degree line as a baseline, both Zynga games (Mafia Wars and Yoville) would appear above the line and infer that they are better at getting their users engaged on Facebook than their competitors.

So while a game may wane based on game play, like Pet Society which clearly shows that it’s beginning to lose active users, it very well could be that the active engagement of the users with a game(and possibly the way that Zynga does it) could be actively extending the typical lifetime of a game application.

Interested in looking at how we can apply the FLAPI to other brands, including those that have only a Facebook Fan page and don’t have an application. And also how much the FLAPI is impacted based on these brands having multiple fan pages – today Facebook Lexicon does not appear to mine those pages (”Lexicon shows the number of users that posted each term per day on a profile, event or group Wall.”), but those pages could be driving user posts.