Do both. I have over 5000 pictures on flickr right now. If flickr fails (you never know) then they are on the HDD, but if your HDD fails then they are on flickr.
Note that free flickr users can't upload unlimited pictures.
I do both, but remember that if you want to keep your photo's in high quality, high resolution on flickr then you need to register ($24 a year, I think).
Rather use a disk for backup and the internet for sharing.
