[wlesieutre]

Yeah, this worked fine for Gmail's early years because you can talk to people on other email systems. But Google tried to apply the same system to both Wave and Plus where the exclusivity made them comparatively useless.

I think they've learned the lesson by now at least, since their latest communication systems (Duo and Allo) had open signups. If those two fail it'll be because they were pointless (from a user perspective) entries into a crowded market of established players who already had network effect lock-in working them.