The ratio of 'do' follow to nofollow links is not a useful indicator of site quality, as far as I know.
More important is the rate of increase, and it seems likely to me that do follow and no follow share different rates. Links acquired through social media or guest blogging are typically nofollow links. They're easy to come by, enjoy little SEO value, and can probably be accumulated safely at a faster pace. 'Do' follow links on the other hand are generally harder to earn, require more effort, and so accumulate more slowly.
If your link profile suddenly saw an increase in 'do' follow links, without a corresponding increase in nofollow links, I think that would look very suspicious. To play it safe, let your naturally gained 'do' follow links dictate the pace at which you use social media to gain no follow links.
Link density? What an odd term. How could it be determined by comparing no-follow from follow ... seems downright silly to me. My answer to the poser is to not wate your timr worrying about this type of junk and concentarte on not manipulating the search engine.I suppose that whatever happens naturally will therefore be the "density" in your case. Is that good or bad for ranking? Who cares really.