I did report this shortly after commenting above and they said they had other reports and they were still looking into it and would update when they figured it out. I'm wondering if with the new update that the "Publish my activity on Spotify" preference defaulted to OFF instead of ON like it had been and that is why everyone's activity is no longer showing. When i check mine that preference was defaulted to OFF. I'm assuming they could fix that since people aren't aware by doing an update and having it default to ON if that is the case and people turning it off if they don't want it. But i thought that was the whole point of choosing a private session. I'm not sure but do know they did state they would update in the community when they figured it out. I suggest checking your Preferences and seeing if what i stated above is off or on... it may be the same case with all your friends as well and why we aren't seeing anything now.
reported + tweeted at spotify a couple weeks ago when i first noticed this happening..still a persistent issue where only a few people will update. checked my boyfriend's account (won't update on my friend activity feed) and he's still sharing his activity so seems like it's a problem with the program itself.