There are a couple of things I have noticed that can be changed with SO or the Stack Exchange sites in general, in order to attract users who are interested in the sites but have slowed down in activity.
For example the weekly newsletter that I receive from SO can and should include a section where it lists activity related to me; profile views, comment votes, comment flags, answer votes, answer comments, and questions that might be related to the questions I have asked.
This, of course will require the newsletter to be more personalized rather than generic:
and would require more effort because it would change per user. However, I think the feedback would be positive; if a user has started showing interest in the site, however, has been swamped with other activity, receiving just another newsletter would not be so annoying to the user because it would be rather more personal.
Additionally, I think a section of the user StackExchange inbox can be dedicated to reputation changes. This is again to motivate users to see what have happened and why. Currently the reputation management per user is spread over the sites the user have registered and combining all of them (as a reputation center) would help the user see which answers, questions and comments have attracted most attention and why.
I think with a reputation section, the user no longer has to go hunting for his/her activity to see what the community thinks of them. For super/expert users a setting can be in place to remove/quiet this section so as to not be annoying.