The groups on Facebook are very different but it still works. You should be able to build most things on top of the platform. Flickr was the social network for photographers but most photographers post on Facebook now.
Strangers don't belong in that model because they are not friends. Most people want to keep strangers away from your real life until they become friends so having a third party site keeps that experience away from preying eyes. Once a couple is formed they will connect over facebook and start sharing their experiences.
You might be right though, it might consolidate in the future if people figure out really good interfaces that make it work well for many use cases.