If anyone ever tells you to eschew building for the mobile Web and only worry about native app development, you might want to be wary of taking the rest of their advice.

I was just over at Facebook HQ for a quick meeting with a small chunk of the Silicon Valley press corps. While it was mostly to discuss their mobile strategy and wasn’t intended to be very news-heavy, they did mention one detail that I (and just about everyone else in the room) found particularly interesting: Facebook sees more active monthly users on their mobile Web app than they see on their native iOS and Android apps combined.

You can see a rough representation of their monthly usage on the graph above. While they (intentionally) left out any hard numbers, they did mention that the graph was to scale and was measuring monthly active users.

Things to note:

  • Facebook sees slightly more monthly actives from iOS than from Android. While the graph above says “iPhone,” they confirmed that it should read “iOS” (in that it includes other iOS devices).
  • They see more users on the mobile Web than they do on either platform individually. Heck, even if you combine the two platforms, the mobile Web app still takes the crown.

It’s important to note that the “Web” column does include mobile Web browsers users on iOS/Android devices. In fact, I’d wager that most of the mobile Web app’s users were on one of those two platforms. This means that even on platforms where Facebook offers rather in-depth native applications, plenty of users still stumble in through the browser.

So, what can you take away from this? If you’re building a Web service, you absolutely want a proper mobile Web app. Maybe not if you’re Instagram, or Netflix, or something else that requires camera/DRM’d video/etc. functionality that mobile Web browsers generally just can’t handle yet — but if you can build for the mobile Web, you should.