I think you'd be hard-pressed to find a LinkedIn alternative. It's too business-oriented for anyone to care that it's under corporate control. In some sense, that's the whole point.
My understanding of Friendica is that it's supposed to fill a niche similar to Facebook. I've never used it though.
Are you able to independently confirm that the domaincheck container is listening to the right port? Eg
netstat -tunlp
on the host