One of my commenters last week asked about the difference between social bookmarking and social networking sites. I think of social networking sites as ones where you can communicate directly with other members, where as social bookmarking sites are ones that you can add bookmarks to be shared with other members without as much direct communication.
There are sites that are definitely considered one or the other, but some sites offer both bookmarking and networking features. Here are some networks that can be used for both purposes.

Favorite a Tweet on Twitter
Social Networking Features
Follow Twitter users
Status updates to all followers
Direct public messaging through @replies
Direct private messaging to users following you
Social Bookmarking Features
Save tweets in your Favorites to share with everyone
Most people think of Twitter as simply a social networking site. But one of the most popular things to share in the 140 character status updates are links to websites. If you have a lot of followers, chances are you have seen one or two tweets with links to articles that you would like to come back to later.
This is where Favorites comes into play. You can easily save tweets by clicking on the star. This will effectively bookmark the tweets in your Favorites section for later. I’m not sure if there is a limit to the number of tweets you can save in your favorites, but at the moment, I can access over a hundred favorite tweets from a year ago.

Attaching a Link to a Facebook Status
Social Networking Features
Add Facebook members as friends
Direct public messaging on walls
Direct private messaging through Facebook email
Connect through applications
Join groups and fan pages
Social Bookmarking Features
Attach links in status updates
I’m sure most people wouldn’t consider Facebook the ideal place for social bookmarking. But one way that Facebook could be used as a social bookmarking tool is through adding links. Whenever you add a URL into a Facebook status message, it gives you the option of attaching it as a link. These links will be shared with your Facebook friends in the News Feed, and will also be saved on your profile under your Links tab. Currently, I can still access 114 links saved since August of 2008.
StumbleUpon

Sharing Bookmarks on StumbleUpon
Social Networking Features
Subscribe to StumbleUpon members
Direct private messaging through the share button or StumbleUpon email
Social Bookmarking Features
Save bookmarks with reviews and tags
StumbleUpon is a good example of mixing social networking and social bookmarking, although the social networking side has gotten a bit more difficult since the new StumbleUpon update. StumbleUpon’s main focus is discovering websites that others will be interested in and adding them to your favorites. You can use StumbleUpon purely for social bookmarking by thumbing up sites to your favorites – just be sure to tag your favorites with keywords that you will remember to search for later, such as having a collection of WordPress themes or Twitter resources. Also, you can go a bit further by adding a review – like your your favorite part of the article – as a reminder of why you liked the article when you go searching for it later.
StumbleUpon also allows you to connect with other members who have particular interests and share sites with them through mutual subscriptions. In the previous version, you could request to be friends with someone and, once they accept, you will be mutual subscribers. Now, the process involves either messaging someone asking to exchange subscriptions or subscribing to someone else first and letting them know, in hopes that they will subscribe back to you so you can share pages with each other.
Delicious

Sending a Link on Delicious
Social Networking Features
Add Delicious members to your network
Social Bookmarking Features
Save bookmarks with reviews and tags
Delicious is known primarily as a social bookmarking site. There are many great ways to use Delicious for bookmarking, such as bookmarking posts with descriptive tags so you can find them later. But you can also connect with other members by adding them to your network and sharing your bookmarks with them using the Send field.
Diigo

Sending a Link on Diigo
Social Networking Features
Add Diigo members as friends
Direct private messaging
Social Bookmarking Features
Save bookmarks with reviews and tags
Diigo allows you to save bookmarks to your library, as well as share your bookmarks with your friends and in groups.
Amplify

Forwarding a Link on Amplify
Social Networking Features
Follow Amplify members
Direct private messaging to members following you
Social Bookmarking Features
Save bookmarks with clips, reviews, and tags
Amplify is a social bookmarking network built on WordPress that lets you save bookmarks to your Amplog. You can highlight selected portions of the page to be included in your Amplog entry, as well as add your own personal review and tags. Once you have connected with other members through mutual following, you can send direct messages and share your bookmarks with them.
Your Favorite Social Bookmarking / Social Networks
What networks do you prefer when it comes to social bookmarking and networking? Do you like the combination of both features, or do you prefer keeping bookmarking separate from networking?