You unfortunately are coming at the problem from the wrong direction.

The only social network they will want to use is the one with all their friends on it; and for the older generations, that's basically just Facebook.

In order to get them to move you'd need to get their friends to move, and in order to get their friends to move, you'll need to get their friends to also move. It's called the network effect and it's why it's incredibly hard for any non-established social networks to gain much of a market share.

Your best bet (which is by no means a guarantee) is to wait for the latest Facebook scandal to be in the news, and chat to them about it whilst they're watching it on TV. Plus add a bit more fuel by doing the ol' "oh this reminds me of something else I was reading a couple of months ago..." And have some other recent scandals in your back pocket to fire out. Bonus points if you can already establish yourself on something like Friendica, which will allow you to say "yeah I quit Facebook a while ago, the company running it just seems skeevy, I've been using friendica instead for a bit now" or something like that

Then you have to hope that registers enough as a talking point amongst them and their friends that it sticks. But you have an uphill struggle ahead with no certainty of success.