Don’t use Instagram, it’s just a million dollar fad which will dissapear in a few years times. Flickr is a good start and so is Photobucket, those sites have been around for years.
Flickr is an established player. It’s been around for longer, and it’s still used by a lot of serious photographers. There’s no point moving to a different service if you’d have to leave all your friends to get there. Flickr also offers photo sharing groups, which is something Picasa doesn’t directly provide.
Flickr is a good service to post your images. But for some reasons I don’t understand it get’s errors on some images. Maybe that was fixed today.
flickr has tones of members which interest are mostly into photography and designs…you can get a good glimpse and ideas from those users while you enjoy doing the same thing