Problem is, most people don't know they exist, or how to use them!

If you've built your own website, then it might be dawning on you about now just how difficult it is to get traffic.

The days are long gone when you could just "build it and they will come".

The good news is that there are some fairly simple ways to make a noticeable impact on visitor numbers.

These online social marketing opportunities are most effective for people with an existing website who need to increase traffic, although there is nothing to stop you from talking about an off-line business either.

Here are four of the most important sites for marketing:

Digg – Harness the Power of Social Buzz

Essentially, Digg is community news voting site where users pick the most interesting stories. It's also a great way for website owners to drive traffic and build backlinks.

People can "digg" a story or blog post to show their approval, and the more diggs a piece gets, the higher it moves up the Digg ranks. If you get enough Diggs in a 24-hour period, your story can get promoted onto the home page where it will attract tens of thousands of visitors.

Check out the Digg Tools & API section and integrate the Digg button on your site, which makes it nice and easy for people to vote for your content. This is particularly effective if you have written a high-quality blog post or valuable article that is likely appeal to the "geeky," tech-savvy community on the site.

Just submitting to Digg - even if it doesn't reach the home page - can have an impact on traffic and it adds a link that can help bump you up the natural search listings.

eBay – Use Auctions to Drive Traffic to Your Store

As one of the most visited sites on the internet, eBay offers small businesses something extremely valuable: leads.

If you sell physical products, either from a brick-and-mortar or online store, then listing a few of these items on eBay can prove extremely valuable, even if you make a loss on the actual item itself.

Why? Because you gain access a whole new customer base. You can advertise your website or shop address on your "About Me" page, invoices and in your email signature tags when dealing with customers.

If you set up an eBay Store, you can make use of the email marketing function, which encourages people to sign up to your newsletter when they visit and allows you to continue marketing to them in the future.

Some eBay sellers also like to enclose their catalog or brochure with the item before they post it out to the customer. These are all brilliant ways to get your business out there and get more customers.

Running a service business? eBay caters for you too. You can post an ad for your business in the Classified ads section and use it to direct traffic to your website – or direct online buyers to your brick-and-mortar premises.

Remember to provide as much information as you can in the ad. The more you tell people, the more likely they will be to get in touch.

Twitter – Gain Attention and Mindshare

Twitter is a popular service for microblogging, a cross between instant messaging and social networking.

Rather than sitting down each day and forcing out 400 words on some topic related to business, people on Twitter post just a sentence or two whenever they have a thought to share.

For example: "Just received a new shipment of eBook readers, which I'll be uploading later today to my website www.readitnow.com." Or "Why is it so difficult to get through to Paypal phone support? Have been on the line for 30 minutes already!"

For time-poor business owners, this is a fantastic way to spread the word about a new blog post or product, meet other people in the business, and get more traffic in general.

Encourage your customers to "follow" your Twitter updates as a way of keeping in touch. This way you can gain more attention from having a bigger network. And in turn, you can subscribe to theirs for a bit of market research!

Squidoo – Providing good quality information = good quality traffic = more buyers

Squidoo encourages people to share their knowledge and passion for particular subjects by creating a "lens".

Lenses can include articles, items available for sale online, videos, audio, recommended books, music…and much more. There is literally room for a lens on any topic under the sun.

People have written lenses on "why buy organic cotton"; "funky laptop bags"; "make your home energy efficient"; "a tribute to Monty Python"; "The David"….and on it goes.

So what does all this actually achieve then? Well first of all, Squidoo gets a lot of traffic and some lenses get a very high page rank from Google. They may very well appear higher in the search engine for keywords than your own website!

Secondly, Squidoo traffic is high quality traffic and thus more valuable, simply because Squidoo visitors tend to be looking for good information and so are more likely to make a purchase.

Squidoo lenses are very easy to monetize. You can link to your own website, promote affiliate products, or sell some of your own range.