GoogleTalk once federated with XMPP/jabber, good times until their userbase was big enough to deferedate again, crippling the jabber network. It will happen again if we let it.
Metas plan is to draw users into their network and use the fediverse as an initial catalyst (“look! so much content already there!”). Once their userbase is large enough, they will deferate again claiming protocol difficulties or something equally vague, but they will just want to start rolling out advertising which would not be displayed to users from other instances. Most users will not keep two accounts and jusy stay with the big corp and leave the original fediverse again.