http://stackoverflow.com/questions/41726463/angular-2-ngif-not-binding/

example.ts

import {Component, OnInit, OnDestroy} from '@angular/core';
import { NavController } from 'ionic-angular';
import {Observable, Subject} from "rxjs";

@Component({
  selector: 'page-home',
  template: `
    <ion-header>
      <ion-navbar>
        <ion-title>Home</ion-title>
      </ion-navbar>
    </ion-header>
  
    <ion-content padding>
      <ion-card>
      
        <ion-card-content>
          <ion-card-title>
            TEST
          </ion-card-title>
          <p>
            User's UID is 1234 and the email is a@b.com
          </p>
        </ion-card-content>
        <button *ngIf="hasFacebook" ion-button>Check in on FB</button>
        <button *ngIf="!hasFacebook" ion-button>Link with FB</button>
        <button ion-button>Logout</button>
      </ion-card>   
    </ion-content>
 `
})
export class HomePage implements OnInit,OnDestroy{

  hasFacebook$: Subject<boolean> = new Subject<boolean>();
  destroy$ : Subject<any> = new Subject();
  hasFacebook: boolean = false;

  constructor(public navCtrl: NavController){
  }

  ngOnInit(){
    this
      .hasFacebook$
      .takeUntil(this.destroy$)
      .subscribe(hasFacebook => this.hasFacebook = hasFacebook);

    Observable
      .interval(3000)
      .takeUntil(this.destroy$)
      .subscribe(count => this.hasFacebook$.next((count % 2 === 0)));
  }

  ngOnDestroy(){
    this.destroy$.next();
    this.destroy$.unsubscribe();
  }

}