Not always. One can use an open base but add proprietary extensions. This is exactly how the web has evolved from Netscape and IE having different tags in the 9

Not always. One can use an open base but add proprietary extensions. This is exactly how the web has evolved from Netscape and IE having different tags in the 90s to the 00s CSS `ms-` / `o-`, prefixes. This is also how XMPP was designed: common base but support for extensions (open or otherwise). Email is a totally different problem because it's a suite of a multitude of different standards used across countless different platforms. At least with proprietary messaging services Facebook et al will still be the implementation standard that most people will use (given that habit has already been well established) but lesser used 3rd party clients wont have to worry as much about Facebook breaking the protocol to intentionally break support for 3rd party clients. However I'd wager you will still see new proprietary features added that will not function in 3rd party clients if just to convince users that the 1st party app is the better client.