This is due to a usergroup change I made last night.
I discovered I'd inadvertently set conversation participants far higher than originally intended so reduced them to align them with the other forums; 1 recipient - newly registered members, 2 - for first usergroup promotion milestone - and max. 5 for longer term members.
A recipient max. of 50 is available via a paid
user account upgrade, but I've never really promoted the upgrade and it is not linked in the main parts of the forum (
it's in your user menu drop-down but I suspect most people don't notice it ... 
) so is not something people have been aware of or have subcribed to in any numbers.
Let me have a think about it tonight and I might make some further adjustments to better accommodate small group chat.
Cheers,
Shaun