I actually originally had something related in my post but removed it: I won't personally consider the site I work on a success unless my mom uses it. She's a physics PhD and she once in a while stumbles onto physics.SE when searching online but she doesn't actually use the site.
I think SO and Server Fault are the size they are because they have an intrinsic advantage over our other sites. People in the target audience work and live in the same tech ecosystem as a website, with something like physics that's an initial burden.
SE itself also has a high barrier to entry. Every one of our sites has it's own rules and definition of what type of content is acceptable. This is for a very good reason, seeking out absolute answers to questions. Most other websites on the internet live to foster discussion, but we're not a forum.
Making stack exchange a common name (in more than our sizable niche) is one of our company goals, but I don't really know what we can do to get past the initial issue I mentioned and reducing the second would hurt our product.
It's quite an interesting idea to be more proactive about bringing thought leaders to out lesser known sites, I don't know if we could do it just directly since most thought leaders are very busy people who already have a dozen places people ask them questions but it's something to think about. We already consider expertise of contributors in Area 51, our incubator for new website ideas.
Perhaps while we may not be able to get the top 1% of thought leaders onto our websites without giving them a really hard question to answer (my mom loves difficult questions, god knows how many times she used to run into my room and say she solved a question I asked her in passing days prior) but it might be possible to bring in up-and-coming thought leaders. A lot of my friends from the health and fitness industry (were I used to live) might want to use our Physical Fitness Stack Exchange site to foster their own personal brand.