FREE OR PAY ACCOUNTS - This Yahoo sponsored photo sharing system of photo groups is truly gigantic in it's scope and scale. When you go to the home page
HERE you'll see a Search field in the upper right. Here you can Search for the Keywords most people attach to their photos, but it also searches for photo names.

Click on
Explore (upper left) and you will see an information page with lots of photo areas to explore. In the search field (upper right) type "groups”, click on "more search types" and when "groups" appears click on that to go to a Flickr groups search page. Flickr groups have names which usually describe the types of photos that are posted in that group. The group names can denote an activity, like sailing or hiking. There are groups named for the cities that photographers live in where you can see pictures of the places where they live. There are also groups for types of photography like black-and-white, macro, etc. Nikon and Canon owners have their own groups as well as specific types of lens owners, like an ultra-wide fisheye lens group.

If you want more flexibility in your Flickr photo postings be sure to check out the Flickr Pro account. The Flickr Help/FAQ section will show you the details on this type of account. It's well worth the modest yearly fee. Flickr has a well designed photo display function. When you go to a groups main page click once on any of the thumbnail images which will display a larger image with discussion threads below. Click again on the image and this will take you to a Flickr slideshow function. Drag your browser window out to fill the screen and with most images there will be a black border. The group discussion threads can be a great resource for photo how-to info, photo gear and software.

Another interesting Flickr resource is the list of Groups that each photographer has joined. To see this look on the main photo stream page top right hand area for an icon that looks like three white dots. Hover over this and a drop-down menu will appear where you can click on Profile. Scroll down that photographers Profile page and you will see a list of the Groups that they have joined, click on Show More at the bottom of the list to see all of the groups. I have found photo Groups in these lists that were about subjects that I didn't know were on Flickr. An additional interesting resource is to click on the photog’s Favorites tab at the top of the main page to see those images-another source of inspiration.

The pdx[portland] Flickr group has monthly meet-ups (taverns/brewpubs) on a fairly regular basis, also some group photo-walks. This group also has a presence on Google+ HERE.
PAY ACCOUNTS - From my perspective
this photo sharing site has it all. A great reputation, Web 2.0 site layout and a great looking Flash based slideshow feature. It offers online printing (Mpix and others) and sales of your photos as well as many other features for the novice to professional photographer... check out their Features page HERE.

photo sharing site has many of the same features as Zenfolio and is equally liked by most photo geeks on online photography forums. If you look at its feature page HERE and compare it with Zenfolio's you'll see they are not that different. I prefer the page layout of the photo galleries on Zenfolio and the fact that they use Mpix for online printing.

photo sharing site has been around for a long time and has a good reputation. The features offered on this site IMO aren't up to the Web 2.0 standards of Smugmug and Zenfolio. If you just want a basic online photo sharing site and a secure place to store photos which can be linked to from online forums, then this may be for you.
high-end photo sharing site has memberships offering different levels of service. This isn't the place to post smart-phone photos of family events, it’s a place for high-quality pro-level photo sharing. They offer photo groups, a forum and blog. Also they offer photo books and fine art print sales.
high-end photo sharing site that has both free and paid memberships offering different levels of service. Their high-quality pro-level photo sharing is organized in different groupings and viewing themes. They also have a blog and offer photo print and download image sales.