Sorry - I didn't mean to sound like I think you're lazy or creating low-quality content. It's just that I see this stage as a natural evolution in a site's success, and a critical decision point. When the low-hanging fruit is gone, you have to start climbing, and that's going to take some more effort.
I just want to say that every big blog started off as a small blog. Granted, if I post on SEOmoz, I get a lot more attention than other places, but SEOmoz started as Rand's blog and built up over years. My own company blog has about 1,000 subscribers (not SEOmoz's 100K), but I've had some major content marketing success over the years, and some of those still generate traffic/links 2-3 years after they were written.
In most cases, those successes not only took an investment in the content (I estimate about 20-30 hours for a really good piece of content), but it took pounding the pavement. I had to build relationships online, find my audience, and generally promote myself and my content. That goes well beyond just posting on FB/Twitter. You have to seek out the people who are the right audience for that content, even if it's one by one at first. It's tough, but once you've built that base of relationships, they'll keep paying dividends.