yes and as far as I know there is work being done to make private channels a thing. So in the near future, private conversation might be able to include more than 7 people.
I just have this feeling that people sometimes talk about SSB as if it is like "private by default" and thats not really the case, public feed is quite public (and I don't think that is bad).
I for example considered using private messages to myself as a diary. I know they will remain private.
You could send a private message to a group of (7 or fewer) friends and then only they could open it.