I'd first start with quite a strict split between: 1) advertising on social networks and, 2) communicating on social networks.
Let's start with advertising. As Moon Ahmed mentioned, you have three primary networks you could advertise on, Facebook, Twitter and LinkedIn, and each of those has their own advantages and disadvantages. Facebook is brilliant as you can use very precise interests targeting to reach a killer target audience. You can also use the so-called Custom Audiences to advertise to your current clients and Website Custom Audiences to advertise to anyone who visited your website - those will be incredibly targeted people and you can remind them about your services. As for Twitter, you will need to test many other B2B business handles to check if you can raise interest among their audience. And LinkedIn - well, that is the absolutely best tool for making B2B connections and you can truly find businesses and people who are in your target audience. However, LinkedIn ads are the most expensive from the three, so be ready to pay as much as $5 for a *click* and potentially even more. Even if it's expensive, it sure is worth trying out.
Taking a step back, instead of just promoting your services with ads, I'd really focus on lead generation. An absolute must in these situations is a strong lead magnet that you would then promote - it can be a free guide, an ebook or even a free live webinar that would solve your perfect client's problems.
Now, talking about the communication, this is were the going gets tough for B2B companies. First, you MUST understand your target audience and the network. As someone who's worked with plenty of B2B to B2C companies, I can tell that it is very, very difficult to promote B2B activities on Facebook. And the main reason for that - people do not come to do business on Facebook. Let me repeat that - people do NOT come to Facebook to do business. Instead, they come to engage with their friends and families - to be social. So that's our primary goal for Facebook communication - we promote less of our B2B services on Facebook and more of the actual people. That builds trust and makes stronger relationships. And that's what I'd suggest for you - introduce your employees, show your office, what you are working on, where you're participating, etc, and people will trust you more.
As for Twitter, their search is the most powerful thing ever. Just use it to look for people that are struggling with problems that your business can solve, and jump in to help them.
And finally, LinkedIn. So far I've been very happy with LinkedIn and that's where we've taken our entire B2B communication to. The spreadability of ordinary posts on LinkedIn is amazing, and even with just a few hundred followers we can reach thousands of other people. As people go to LinkedIn to do business, the communication fits right it.
I hope that helps! If you'd like to discuss this topic even further, feel free to give me a shout.