I'm a french web developer & web marketer, I currently live in the Philippines where I work with small & medium businesses, assisting them in their social media marketing & SEO mostly.
I generally recommend a mix of Social Media Marketing & SEO to my clients, it has many advantages in my opinion:
SMM will be faster to grow and will provide audience while SEO is still starting.
Social Media Links are part of a natural link mix & shares/likes/+1s/comments... send strong signals to search engines so it definitely helps for SEO.
You don't put all your eggs in one gasket. :)
I agree with Rayvensoft about the engaged audience, target quality & relevancy, not quantity.
One good idea I've seen for authors is to find influent people on social networks and propose them a free copy so that they can review it, it's a simple way to get free advertizing.Aside from that, I invite you to read Seth Godin, he published many best sellers, changing his marketing every times, there is a talk on Ted.com in which he talks about this also.
You should explain what you already did so that we can tell you what to add to the mix.
What you need is an audit I suppose, but I give some tips anyway, the ones I gave on this discussion are also good I think.
Use a better html structure with logical h1, h2, h... strong tags
You should host your website in Germany.
Re-think your categories pages (example: http://www.akkus-adapter.com/bosch.html), it's not easy with that much products but it really looks spammy like that.
Embrace Social Media, be active in building a community around your brand.
Add a blog on which you'll talk about your products, IT news... It'll be the cement of your social media activity.
Read my blog if you like (see my sig). :)
First, you need to update your signature, you forgot the "http://" obviously and the link won't work.
Second, concerning your website, here are my thoughts:
Congrats for the design, I really like it. The content is plenty on the products pages, it's a really good job.
On the speed optimization side, you should merge your css & js files if it's possible without too much hassle.
You should maybe remove the icons to the social networks/bookmarking sites you don't use in the footer.
Your Google+ link is wrong, you can find how to get the correct link here.
Good luck with your website but I'm sure it'll work.
You're in a very competitive field but you can try these:
Audit your website & optimize it for SEO
Mike's idea is good for sure.
Develop your presence on social networks, interact with others & help them
Answer on forums and websites like quora.com, stackexchange network... to gain authority.
Publish articles on other websites with a broader/complementary audience.