On Facebook and Google Plus I usually share my website posts on my fan page and groups related to my niche.
On Twitter I usually relate to people interested in my niche. In a simple keyword search I get to know what people's interests are and that interaction has worked positively for me.
I do not use Instagram because it is not part of my strategy, but I also consider it an excellent social network.
I think the secret to succeed in social networks and have quality content, good titles and flashy images. The use of videos and gifs is also very valid. In addition, the frequency of postings is also important. I usually make a single daily post on my social networks, but that will depend on your niche.
I seldom do paid advertising. I focus more on the organic side of marketing then if I observed that a certain product needs boost, that's the only time I use paid advertisement. focus more on your target market. what age group, location and hobbies do they like then utilize your social media platform especially Facebook. Use your personal account first then create a page bor group that you can market or attract more consumer to your product.
Sharing of one's website and post is the secondary aspect of using the social media networks to generate traffic. The most important thing is to first build your fan/friends/followers base before anything else, because if such people aren't already in place, sharing your website or post would simply be like pouring water on a rock which never penetrate into it.
It's when these fans/friends/followers are well stocked and impressed with the content you normally offer on the social media platform, then when you link up stuffs from your website to generate traffic, there is a 100% chance of having all of them flock to your website and increase your traffic.