Web 2.0, in my opinion, has and will continue to affect SEO. (There is no single definition of a Web 2.0 site or property but one thing almost EVERY site that is considered to be a Web 2.0 site has in common is they are interactive.)

I’ve also read an article at Search Engine Watch where the author stated his belief that Google will soon adjust their algorithm to give more authority to sites with:

Lower bounce rates
Incoming Social Bookmarks
RSS Subscribers

Bounce Rates

The bounce rate of a site is high when you have a lot of first time visitors and not much in the way of repeat visitors. This tells the search engine that your site isn’t very ’sticky’. As the search engines see it, you aren’t filling the needs or wants of your visitors if they never come back. This may or may not be true but that’s the perception the stats give.

Social Bookmarks

When you have buttons on your site so visitors can add it to their favorite social bookmark site, you are giving them the opportunity to tell the search engines they LIKE your site. I believe this positive vote WILL be counted in the near future.

RSS Subscribers

RSS is an acronym for Real Simple Syndication. This is a technology that has been around for years but it has just recently begun to be adopted by many webmasters and users.

The power of this is that someone that likes your site can subscribe to your RSS Feed and they’ll be notified every time you add new content that the feed picks up. (WordPress automatically adds every post to the RSS Feed.)

How SEO is Affected

If you look at your stats and see that you have a high bounce rate, this means your market research might have been too narrow, you might not be offering a wide enough range of products, you aren’t adding new content often enough or the keyword phrases you chose didn’t meet the searcher’s expectations once they saw your site plus other factors.

You can and should do SOME social bookmarking of your pages BUT many social bookmark sites prohibit self promotion which is a good thing. Natural bookmarking by your visitors not only creates the valuable inbound link you want BUT it also gets your site in front of THEIR friends without you doing anything. This is word of mouth advertising. (Social Bookmarks are stored in categories and by tags. Think of a tag as a keyword phrase.)

RSS Feed subscribers can subscribe and unsubscribe without you knowing it unless you are using an RSS Feed tracking service or some other tracking program that will tell you how many subscribers you have. Each subscriber is an unspoken testimonial that someone likes your content and WANTS to be notified when fresh content is available.

Some Web 2.0 properties are:

YouTube

Squidoo

Hubpages

Flickr

Blogs

Article Directories

Social Networks

Social Bookmark Communities

The above sites cover video sharing, image sharing, article sharing and journals written by real people that often don’t think about making money.