I have tried both types of services. The open-ended "directories and social media sites" are not worth it. Honestly, I would hire a college intern looking for SEO experirence and a possible future job and train them the way YOU want it done. Meaning, high quality relationships built, real emails sent with personality, guest posting articles that they can create, etc.
It would be much more value than the bigger open ended companies, unless you get a great recommendation from someone on who to hire.
The truth of it is that those bigger type companies who you would be paying are also hiring the cheap interns who don't care about your business and don't know who you are. If you hire them directly, at least they will care more.
Good luck!
