Some good points.
I suspect that anything open source is going to start as indie, small, and messy, and from there might grow in to a viable open source alternative.
I think that it would not necessarily be better for something to kill facebook because it would then replace it, and most likely be just as 'bad.' It would be better perhaps to have viable alternative, the competition between them would then force each to maximise the quality of what they offer.
I guess in principle that is what g+ could be considered to be, what ever you think of google. I've managed to avoid g+ so I can't comment.