Which Facebook apps are people really using?

User engagement can be a very slippery statistic. Although sites and apps like to talk about how fast their unique visitors are growing, it’s more important to know how many of those users are actually using the site regularly.

Using AppData, which measures the usership of Facebook apps through the Facebook API, we determined “stickiness” for a handful of popular apps. Stickiness is the ratio of daily active users to monthly active users—or, the percentage of monthly users that use the app every day. It’s an excellent way to gauge an app’s popularity. We compared some competing apps that users mainly access through Facebook(s fb) — Viddy and Socialcam, for example, and Draw Something vs. Words with Friends — to see which have higher user engagement. Here are the results.

Viddy vs. Socialcam

Facebook’s recent purchase of photo titan Instagram raises the stakes for the obvious next technology in line: social video. Viddy and Socialcam, both launched over two years ago, are battling for usership, brands and celebrities. With 6,600,000 daily active users and 54,100,000 monthly active users and a stickiness of 12 percent (compared to Viddy’s 1,500,000 DAU, 35,000,000 MAU and 4 percent stickiness) Socialcam is clearly in the lead.

Draw Something vs. Words With Friends

Zynga(s znga) bought OMGPOP, the maker of Draw Something, in March. Since then, Zynga has started selling prime ad space on the app, and Draw Something’s user engagement has dropped to a still very respectable 25 percent. But not nearly as respectable as Words With Friends, which is at 40 percent — so that means, almost half of its monthly active Facebook users are spreading their words daily.

CityVille vs. FarmVille vs. CastleVille

Zynga has been making some big moves lately. This month, it joined with American Express for a Zynga Serve Rewards card, which offers users Zynga Farm Cash as opposed to more traditional rewards like actual cash or air travel.  Zynga also hired a Washington lobbyist firm to effect change in the real world as well. In its popular “ville” series of games,  CastleVille leads the pack when it comes to user engagement.

Readers: What other app pairs should we compare?