Use wiki vs. eVite or Facebook for Networking?

Unsure if I need a basic or upgraded & invoiced wiki... The wiki concept seemed to fit my purpose for a grass-roots business networking process. I only need 1-2 editors to publish information about upcoming event, but would like the 400+ interested people to be able to suggest themes, locations, discussion topics. We have no budget (since we are grassroots), so certainly could not address $x / user / month.

Does a wiki fit this purpose or should I be using Facebook or eVite?
 
sad I’m confused
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