Discoverability. Facebook has it inside the app. Email and texting require you to go out of band for it.
It's impossible to exaggerate the value of this. Groups of people who had lost contact in high school have gotten back in touch in retirement, planned trips, and traveled to a place to meet up together because Facebook has in-band discoverability. Before Facebook such a group of people would just hope someone in the group had enough time and patience to manually track everyone else down, which was probably hundreds of dollars of their time wasted.
Musicians find gigs through it. Friends of friends are leaky so you don't even have to bother people to find useful connections or fan pages. Businesses list specials, trips, blogs, etc. Way cheaper than sending out a snail mail newsletter or even managing an email list.
All Facebook's users save oodles of time connecting with acquaintances and making new connections, for whatever reasons they wish to connect.
Honestly, it's an incredibly difficult question to answer fully because I'm not sure where the short circuit is. Why do you see value in messaging protocols but not social networks?