Of course, Microsoft itself bought into many markets once upon a time. While Excel isn't one where that was the case, its sister app PowerPoint started as Microsoft's first major acquisition.
The point is, you can have that vision for a while, but eventually the market changes such that the threat (or opportunity) to your business isn't so direct to be obvious like Instagram was to Facebook.
Yup. Classic example of innovator's dilemma. These are the opportunities startups need to constantly find and solve.
Proof that simple ideas seem so simple on the surface yet become these big opportunities. What you do with the idea/initial success is what separates these successful companies.
I think he already did and bought Instagram & Whatsapp. Yea real threat is when they miss the opportunity.