"How do I talk to my friends and neighbors about Lemmy?" No, literally I could use some one liners. My friends aren't tech savvy so when I bring up the Fediverse their eyes glaze over.
The trick in conversion is that you want to pull people into your service, rather than pushing your service onto them.
To use a sales analogy, imagine you're shopping for a car. You visit a salesman who's very pushy. They tell you all the reasons you need to buy this car, insist that this is the best car for you, and put you in the car. Then you drive the car and realize it doesn't meet your needs. You might think the salesman was terrible and vow never to visit them again.
Conversely, imagine you meet a very different salesman. They let you walk around the lot, answer questions about each car, and maybe even talk you out of extra features you don't need. "Yeah, the SUV will have more room for groceries, but for your daily commute the fuel economy is going to bite you." You'll probably respect that the salesman is trying to meet your needs, and the next time you're shopping for a car you'll visit them again.
Getting people onto the fediverse is somewhat similar. We're already here, and if you've been here for a long time then there's probably a bunch of things you like about it. You might insist that your friends should join lemmy because it's open-source, ad-free, aligns with your politics, decentralized, or whatever. But if you're leading with all that, then you're putting your values onto the person you want to join. If those values don't align, then maybe they'll join, but they'll just as quickly leave if it doesn't immediately mesh.
When I want to pull someone in, I'll mention that I saw some meme / post / video or whatever on lemmy, and then tell them about the post. You're leading with the value of the content, and if the person you want to join also enjoys that content, they might ask what lemmy is. This is your invitation to pull them in.
Okay, so you asked for a one-liner, and here it is: "It's like reddit, but it's free from ads and corporate ownership."
If they seem interested, give them a link to whatever instance you're on. A lot of the time we might want to talk about the whole decentralized thing and picking out instances, but if you know nothing about the fediverse then the process sounds confusing and can turn people off. Just a link to a registration page will do and they can take it from there.
When I want to pull someone in, I'll mention that I saw some meme / post / video or whatever on lemmy, and then tell them about the post. You're leading with the value of the content, and if the person you want to join also enjoys that content, they might ask what lemmy is. This is your invitation to pull them in.
This. Marketing 101. Tease with content, not with technology.
As others have said, email is a great way to get the point across without getting into nerd shit. I just say something like this: "You know how you can have an email account with Microsoft and can send messages to an email account at Google? Lemmy is that, but instead of sending and receiving emails, you're looking at, voting on, and leaving comments on posts like you would on Reddit."
I have had a 100% success rate with this analogy when the person I'm talking to knows what Reddit is. If they don't know, then I don't really bother explaining it since it's likely outside of their interests. I'm not out proselytizing for the Fediverse, but it comes up on occasion (especially when I share a links to shitposts).
I think it's generally a bad move trying to explain the technology first, it's how people tried to advertise Mastodon when the Tumblr exodus happened and I bounced straight off it. I looked at the list of instances and they were all stuff like "a Mastodon instance for sci-fi nerds" and I couldn't get over the inertia of tying myself to that designated interest.
What we need to do is get people to create accounts and experience the fediverse and then we can point them at examples of the tech if they want to know more, stuff like "that person you replied to is on a different instance and you're able to speak to each other because of ~federation~".
Don't pull out the techno centric paranoid fanatism front and center. Just tell them it's a different flavor of cult that is generally much harmless and funnier than the alt-right ones.