Today's grandmothers won't be around for long. The core generation that joined Facebook because it was cool will become boring old people themselves, and that's why Facebook won't survive.
My fiancée and I are in our late 20s. We were in college when Facebook came out and arrived at the party early. We built our social networks to share the fruits of our newfound "adult" freedoms- namely pictures of inappropriate Halloween costumes and drinking games.
Now, Facebook is different. My mom is on it. My mother-in-law-to-be is on it. But because my fiancée and I are on it, our children likely won't be. It won't be a fun place to share things you don't want mom to know. Mom will be checking in and leaving embarrassing messages on your wall. I think my kids will probably find somewhere else to hang out. It will probably have a name I can't seem to remember, and I will likely embarrass them in front of their friends by pronouncing it wrong or misunderstanding its key features.
Social networks are binding, but they're highly generational. Unless Facebook can figure out how to get my future kids to think its cool, it's toast in 20 years or less.