This is a common question:
"My members won't interact on the site.They interact via e-mail but refuse to use our super expensive, brand new, feature-filled, community site."
The words in italics are mine.
Why does it matter where people interact? It matters if they're interacting.
Build your community in the easiest place to build a community. By forcing people to use your community site, however expensive, you might prevent a community from forming.
If people only want to interact via e-mail then use e-mail. Create a simple mailing list. Let information be easily exchange on the mailing list.
Better yet, use a platform that automatically syncs with e-mail so people can subscribe to a particular category and use that platform. Automatically enroll existing participants on the list with an option to unsubscribe. Summarize some of the best discussions and post them on the site for posterity.
Soon you might just find people visiting the community because it serves as as useful resource.
As a bonus, off-site activity allows further development of the community. It allows real connections to form. It makes you less vulnerable to any platform problems.
People want to use the simplest communication tools. Every extra click reduces the chances of activity happening. You can fight it or embrace it. If people are interacting somewhere, build the rest of the community around those interactions.