Social signals is an absolutely safe way to get natural backlinks. Now social signals are somehow correlated with search engine rankings, especially it concerns Google+ and the authorship markup, and probably the impact of social activities on rankings will be only growing in the near future.
Article marketing and blogging (including guest blogging) are good methods too, because they allow using backlinks in the most relevant context.
For local SEO use co-citations and co-occurences, which are close to contextual backlinks in a way and appeal to the latent semantic indexing too.
Also, product/business reviews and press releases are beneficial both in terms of backlinks and customer outreach.
Overall, try to get backlinks only from relevant sources and build meaningful and useful content around the links, that's what quality backlinks are about.