One of the things that Facebook does very well for brands and companies is gather and present data about the people who are affected by the Facebook page. From finding out how many total friends your fans have to how many people saw your individual posts, there is a lot of information that can be learned about who is interested in your product or service. This week, however, the site admitted that the information they have been reporting has had incorrect numbers for some of their metrics.
Fortunately the data is not affected, but merely the reporting, for impressions (how many times the post has been loaded, regardless of duplicates) and audience reach (unique number of people who have seen the post). Since the data is intact, that means Facebook can patch the system to restore proper data. They are in the process of rolling out the new code, with completion hoped for the end of the weekend. As of writing, the numbers for our fan pages have not been changed.
So, how did Facebook discover the issue? It happened during a comprehensive audit of their system and its data. They noticed a discrepancy in reported versus stored data and set off to work looking for the issue. Luckily they seem to have found and fixed the issue (or so they have reported).
If you are a brand manager, you should start looking at your data on Monday to see if your numbers have changed. My guess is that everyone will see some fluctuations, but larger pages are much more likely to see bigger swings. At least this means you could be doing better than you thought! This goes to show that even with thousands of developers, you can still make a mistake.