I agree with you. I clarified similar position in a followup[0]. However, let's not just confine ourselves to grandma and personal pictures. The issue is all encompassing content; some of lesser value, and some greater. You may think all of your shared content is of value, but the receiver may not feel the same. You have to judge (think) before you direct your message. It is of course an easy choice to send baby's pictures to the grandma, but when you have to target different pictures, you have to think about the target audience. Going one level deep, you may have a story to share instead of a picture. You are now hard pressed to email because you know others may have already seen it, or something else.
Ultimately, the evolution of the web to the social media has happened due to human's lazy nature. We are not capable of directing messages of mass nature to one-on-one. We can direct messages of personal nature to one-on-one, but that's about it. For this reason, social media - versus email - has been able to make inroads into people's lives. Certainly, we feel that one-to-many is not the right approach, hence we form this or that group, to confine ourselves into self-created boundaries (groups have its own pros and cons).
[0] https://news.ycombinator.com/item?id=12345534