Currently when you share a playlist to Facebook/Twitter, it shows in your activity feed (and that of your followers) like below:
The numbers shown, in this case "5" and "11" correspond to the number of followers the playlist has, although that is made in no way clear when you view the Activity feed. Since it is not possible to view all of the playlist followers anyway (clicking the number does nothing) I think it would make much more sense to display the date/time that the playlist was last updated.
This idea has been submitted more than 1 year ago but unfortunately hasn't gained enough kudos during that time (10 kudos/year). In order to keep an overview of the active ideas in this forum, this idea will be closed for now. Note that this does not mean the idea has been declined by Spotify.
Please feel free to re-post this idea if you still feel you want this implemented. With a little change in presentation, maybe this time it will gain the necessary support from the community.