I like in the 7th point in this list of 7 more reasons why web apps fail

The amazing thing about Flickr is that nobody uses the service to upload pictures. Nobody says to themselves “I need to upload me some pictures”. Instead, they’re satisfying some other need in their lives, like showing off the new kid to relatives. Or showing their friends how their trip to Europe went. Or letting their co-workers in on their conference activity. All of these things have to do with their life, their relationships, their everyday activities that aren’t centered on the Web, but are made much easier by it. If we look closely, that’s what most successful web apps do: they make our offline lives richer.

I believe that if technology is both easy to use and helps people in their everyday life then it naturally becomes a success. Email is a killer application in doing this. Flickr does that brilliantly too and that’s why they’re so successful.