One of the things I'm curious about is given Zuck's reputation for childish behavior and jealousy if alternative links will even get outright censored in the feed (or even through Messenger?!)

Google+ "failed" (for various definitions thereof) and while it had numerous failures of its own I wonder how much critical mass it failed to achieve because Facebook was hijacking the sharing of it

Like they hijacked email addresses, to make it hard to move them to another social network https://news.ycombinator.com/item?id=4151433