One of the best examples and my personal favourite is Flickr.
I've been using Flickr now for a number of years and I find it to be quite brilliant. I even pay to have a pro account on Flickr which enables me to upload as much as I want and as large as I want too.
I think it's a great way to keep and organise an online portfolio which you can link clients too. I've linked my own domain name up to this aswell so that when someone types in http://www.alexandermoore.co.uk/ on their address bar they are sent directly to my photostream on Flickr. This is useful because that is the website that my business card states and I can change where that domain name points too if I was to get my own dedicated website online.
The bad things about Flickr are, although you can surround yourself with some very skilled practitioners and find some great inspiration. The community is over run by amateurs who upload a lot of poor or irrelevant images, for example, lego or dolls in an unartistic fashion. It's almost like they are trying to run a community about something else within Flickr.
Another serious issue is that it's not 100% suitable for clients. When you link a client to your Flickr, they can see the work from a whole host of other photographers and this looks very unprofessional and quite distracting also.
In Short Flickr is a nice large community where you can share your work and observe other peoples and breed creativity and inspiration. However it's not a professional tool and is more aimed at amateurs, this will probably prevent clients from seeing you as a true professional. After all, a professional needs their own website, right?
