Thank you for the compliment, and I agree that users can indeed do a lot to boost Cake exactly as you suggest. I'm just not likely to do that right now.
That probably sounds ... ah, "blunt" at best, or "like I'm an a-hole" depending on how it comes across, but I'm genuinely hoping to be helpful by being sincere here. I'll try to explain.
So, yes, absolutely, getting users to draw in other users is key. The trick, I think, is that framing it in terms of "think about how you can help" is expecting your users to perform free labor for you. It takes work to brainstorm about how I would help a for-profit company. It takes a modicum of additional effort to go drop those links around all other social networks when I could have just posted to any of those networks in the first place.
Now, there are a few ways that companies do manage to get their customers to do this kind of work on their behalf:
1. Encourage a sense of community or subcultural belonging. This one has worked wonders with "geeky" entertainment media for years. (See: Textual Poachers, comics fans going out of their way to support favorite books and creators, or, I guess, chapter 5 of my dissertation, with apologies for how verbose it is ... but you probably could've predicted that from my posts here.) This doesn't just have to be a sense of community with the company itself, though, especially when we're talking about a social site. You folks at Cake are super nice, and I like you personally! I don't have a deep emotional connection to anything created here yet, though (like fans have with their favorite comics/shows/movies/whatever), and I have yet to find a community I recognize and belong to here. Absent these things, I am not quite personally invested yet. In contrast, I've been frustrated with some creative and marketing decisions Marvel and DC have made in recent years, but I love their characters, and their stories are cultural touchstones for communities I belong to, so I keep watching their movies and shows. And I kind of hate Twitter as a company, but it hosts too many communities I feel connected to for me to walk away from its service right now.
But of course this is the point we started at—it's a lot easier to get users to do promote your thing when you already have plenty of users. So, next thing....
2. Build a sense of shared ownership. As you say, I already think about how I'm going to promote my blog, but I'll do that work for a post on my own blog because I feel like it's mine—I can track analytics on it, I could monetize it eventually if I wanted, I could export it from WordPress.com and re-import it onto some other host. But I'm not really inclined to put in that extra effort for Cake because right now, to me, it's a cool feature set with a friendly and responsive team who I want to succeed—but I don't own this space, and I am well aware that it is a for-profit service. (This can cut both ways, though; again, see Textual Poachers, and note the example of Star Wars fans feeling like they have more ownership over the films than Disney/Lucasfilm.)
I am not sure offhand how to encourage this in a positive way (and doing the research to figure it out would be the point where this transitions from "I have some thoughts I hope you'll find interesting and helpful" into "let's discuss my hourly rate"). I do think, though, that you are doing better than average by so openly soliciting user feedback and responding directly to people, which definitely a component in this strategy. It really helps users feel like they have a stake in the product and some say in the direction that it develops. And hell, I guess it's working at least enough to get me to do some "work," because what else do you call writing a thousand words on a weekday?
Still, I think there's another major thing that could be pursued—maybe already is being pursued, and I just haven't looked deeply enough to see it yet. And that is....
3. Make it less work for users to promote you. Instead of asking users to think about what they can do to get more users onto Cake, don't make them think. Make it as easy as possible for them to do whatever you want them to do. Give them tools that help you draw in new users and help them accomplish whatever it is they want.
For instance, I once worked at a mobile game company that was looking to leverage social media to draw in new users. They didn't ask people to talk up the game on Twitter and Facebook—they built a "share" button into the game so that when you get a new high score, you can easily say hell yes I want to brag about this, and their Twitter/Facebook account would immediately blast that high score (and a link to the game, of course) to all their followers.
Writing about this just reminded me that I could probably do exactly this with WordPress.com. I popped in and linked it to my (doomed) Google+ account in a matter of seconds. Now it'll automatically share everything I blog to my G+ feed, and comments on that G+ post will be comments on the blog post. It still required some work from me in connecting my accounts, but it's less work than it was before. Could've done the same with Twitter, Facebook, or Tumblr.
To be clear, I'm not saying this specific thing is a thing you need. (Though I will admit that if I could automatically share posts from WordPress.com to Cake, I definitely would, and if I could automatically share posts from Cake to Twitter, I would probably do that too.) Rather, I just mean to say that getting users to do work for Cake will probably require Cake to do some work to make that easier for users. I have to assume you're already doing stuff that could be described that way beyond general quality of life improvements, so I feel like kind of a know-it-all jerk for even writing this ... but I'll hit "reply" anyway because I sincerely hope it helps.
And hey, neat, I see now that the "Reply" button is visible at the top of my screen still even though I'm scrolled way down. I requested that feature on one of my earlier visits to the site, and now, there it is.
This is why I'm willing to write a thousand words (and counting) on this site. I am sorry that this does not translate into a willingness to start more conversations here and share them elsewhere ... but I think it could, in time, especially if it were just trivially easier to do so. I would like that.