I plan to send them an email first asking why their account has been inactive for so long. I won't just yank the plug on them. But on the other hand, I don't want dead weight on my network. I want vibrant members who actually take PART in my site. What do I need with someone who doesn't even care about my network enough to log in once a week, or once every couple of weeks??? They're basically useless! Just a number in my total member count!
I don't see the downside to a large membership list. Some members like to "lurk" and may still gain benefit from being a member. Some of my members RARELY join conversations, but they do like to submit photos and keep up to date and read what's going on on the site. It doesn't cause you any more work to have these members that never participate visibly. That's my POV.
But what you do with the content of the member if the such member has a lot of contributions like in blog post, groups etc. so i think this is not a fair way to deactivate the account with the his/here data. By doing this we will lose the lot of nice & important content from our network. so think about it..
In my case i have the network of my university related students. http://www.vustudents.net . where members contributes a lot in their studies time but after completion their studies may be they do not give a lot of time so it is not a fair way that we deactivate their accounts with data..
I have a "Last Active" app on my site that shows the last time/date a member was active, on their Profile page. The data is stored in an external database. I see how Ning suspends accounts with a removeByUser function, and I could probably periodically call this for inactive users I could get from the database based on some time criteria. But it would be a challenge to test this!