OWA normally posts only 2 months of FB data by default, this is done across all users on the Exchange server. Outlook has the ability to let each user post as much or as little FB data as they need. (Tools | Options | Calendar Options | Free/Busy Options). However since the Exchange server has to do this on behalf of the user, it affects all users on the Exchange server. This can be a problem when an Outlook user has posted 8 months of FB data, then they use OWA to make a change in the calendar which requires the data to be reposted. Now only 2 months of data is posted until a change to the calendar is made using Outlook which then 8 months of data will be reposted.
Two months of data was selected by default for Exchange because that's the default that Outlook posts. However a registry key can be added to have Exchange to post more or less months of FB data.
This key is not present by default.
Add "FBPublishMonth" under the registry key HKLM\SOFTWARE\MICROSOFT\EXCDO\PARAMETERS
This is the number of months of FB data you'd like to publish.
Note that the name is FBPublishMonth and not FBPublishMonths. If this value is set to zero, no data will be published.
Keep in mind this affect all mailboxes on the Exchange server where the registry key has been added, and only affects OWA posting FB data.