Facebook Messenger, Hangouts, Google Chat, and above all SMS / iMessage. Whatsapp by all accounts is the most popular (as much as I personally detest it).
I know I know, not HN-fare exactly, but they're easy, popular, and work. That's all that most of our's social network actually cares about. There's a lot of discussion in this thread about detailed encryption/security policy, but while I am a privacy nut, none of my non-IT friends/family could possibly care less... And they're not wrong. This convoluted encryption which severely limits multi device usability Andb frequently has louse ui is out of proportion with other comms methods such as email also used. While I may stand up for principles of privacy, my family isn't wrong asking why should their exchange of recipes or birthdays wishes be any harder than necessary.