The release of iOS 12.2 and Safari 12.1 changed how first party persistent cookies are handled for Safari users. The expiration of these cookies is hard capped at seven days since the user was last seen. First party cookies set via HTTP response is unaffected by this update and third party cookies will continue to be blocked, as per previous Safari updates.
FullStory uses these first party, persistent cookies to roll up sessions to the user/visitor level. Now that these cookies are force expired after seven days of not being seen, it can cause a perceived bump in the number of people you see in the FullStory Everyone segment, but not the number of sessions.
Safari ITP 2.1 could potentially inflate the number of People (above) you are used to seeing in FullStory, but not the number of Sessions (above)
While most of the effects of Safari ITP 2.1 relate to higher than previous user counts, above, there are two types of searches in FullStory that will yield higher than previous session counts, below. The New This Week Segment and multi-event searches that are across any number of sessions are both affected by this change.
Safari ITP 2.1 can increase the number of People and Sessions for New this week (above, first) and decrease the number of People and Sessions for across any number of sessions searches (above, second) that you are used to seeing in FullStory
FullStory customers that are most affected by this update are:
Sites with a high amount of Mobile traffic because of Mobile traffic's greater percentage of Safari users
Sites with a low amount of identified users because using FS.identify() api does not rely on browser cookies to identify users
Sites where visitors come to the site infrequently because the cookie renews if the user visits the site within a week
FullStory will continue pursuing short term and long term solutions to this issue, in accordance with our beliefs in end user privacy.