For this functionality such systems work okay - but they won't work for kids-to-kids communication due to the same chicken and egg problem.
Your daughters wouldn't (couldn't) use it the way you imagine, because, face it, the 'critical mass' of other kids who'd they want to talk to simply wouldn't be on such a system online/active/quickly responding as they are on facebook/snapchat/whatever.
And I'm not sure if any classmate communication forum with a heaping level of oversight is realistic - the nature of teens and pre-teens is such that any must-be-part-of social activities would happen elsewhere; and if all the good stuff is elsewhere, then they don't need/want to use the monitored forum.
Your daughters wouldn't (couldn't) use it the way you imagine, because, face it, the 'critical mass' of other kids who'd they want to talk to simply wouldn't be on such a system online/active/quickly responding as they are on facebook/snapchat/whatever.
And I'm not sure if any classmate communication forum with a heaping level of oversight is realistic - the nature of teens and pre-teens is such that any must-be-part-of social activities would happen elsewhere; and if all the good stuff is elsewhere, then they don't need/want to use the monitored forum.