Everything can happen if you change the software platform
Depending on the software you used before, XF might require less pageviews to perform the same actions so a decrease in page impressions would be have ro be expected in this case.
vBulletin without Quick Rply being enabled would be such a case, as this requires a new page being loaded for every new post (reply or thread).
A decrease in user activity can be caused by many things - maybe the users do not like the new UI or they were always logged into the old software, and have forgotten their password (which isn't a problem per se, XF has a password recovery function but users sometime have trouble to use that, can't access their email account, etc.)