>>Maybe you could limit non-paying users to only X posts per month, or X messages read per month.
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>Non-paying users also provide solutions adding value to the site

Experts-Exchange has an interesting model - If you pay, you're cool, but if you don't, you have to answer other user's questions to earn your posting abilities. Of course, I'm sure that this doesn't apply to Chatter there (if they do have a Chatter forum), but I think that could apply with great success in the technical forums here. That would motivate me to do a little digging into potential solutions, as I'm not a paying subscriber.