A conversation is hard to have when there's nothing to talk about. For an exchange to really take off, there need to be enough thoughts added to the discussion - 2 thoughts by 2 different people will get you going. Until 2 people share at least one thought each, the Rate Thoughts step will be inactive to preserve the anonymity of the first participant.
Tip: If you have enough time, invite a smaller audience first to help kick-start an exchange before branching out to a wider audience.