Brand page managers are about to see some key numbers plummet as Facebook refines audience tallies.

That’s data about to occur for the audience measurement called weekly total reach, and for check-ins to physical locations. Tallies of mentions on Facebook, which are labeled people talking about this, will continue to be expressed as a running tally, and will not be affected.

Facebook has been sending out messages to page owners with the subject line, “Check-in numbers on pages will be updated in the next few weeks.” Here’s what these missives say:

We are revising check-in numbers on Facebook pages to give you a more accurate picture of how people are visiting your business. Among these changes, previously, if an individual checked into your business multiple times, each check-in was counted into your page’s total check-in number.

Now, if someone checks into your business multiple times within a 12-hour period, that action will be counted as one unique check-in. In addition, when people tag their friends at a location and upload a photo, those photo tags will be counted more holistically.

For example, if 20 photos were uploaded to an album at a specific location, we’ll now count that as a single check-in. If Jessica checks into a location and tags five friends in the photo she uploads with her check-in, the total check-ins number will be six — Jessica plus her five friends.

On the positive side, it seems like this change will, as intended, give page managers a more accurate read on engagement with their pages. On the negative side, however, totals for key metrics that are publicly visible will drop precipitously.

Page managers: Have you received this email from Facebook yet?

Many thanks to Dennis Yu, chief executive officer of BlitzLocal for the tip.