I like the concept of indicating inactive profiles. This could either be done via a public activity counter (as someone suggested show date last active on the site). Or privately, FounderDating could send out a message after not seeing any activity on the site for a certain time (say 2 months). The person is then asked to respond to the message- yes, I want to continue on the site or no, remove my profile (maybe third choice of mark me inactive). If the person doesn't reply to any of these messages, the site marks their profile inactive after 6 months (while notifying in advance). The person must then actively request to have the account reactivated.
I have also noticed that at least 25-35% of my messages are not answered. Then you don't know if that means the person isn't interested or that are inactive. It would be a time saver if I knew ahead of time the profile was inactive so I wouldn't spend time posting the messages.
I guess a proactive step we could take is to search for forum posts from the person. If they have no posts there is a greater chance they are inactive.