Now I want to show user's post count instead of 'Last seen':
Last seen: 2 weeks 6 hours ago
I could not see any guide in the docs so appreciate your help.
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it appears you need another module that will feed this info to author pane. This module is User stats.
Apparently though User stats has only a development release for D7 and reading through this issue, I would be reluctant to implement it on a production site. Will keep you updated if I find a better way to deal with this.
Edit: I've tested the 'post count' functionality of the 7.x-1.x-dev version of User stats and it seems to be working fine.