Remember that web 2.0 by its very nature is ephemeral. Most web 2.0 applications remain in permanent beta - and their functionality and terms and conditions may change with little warning.

As a general principle the more widely used the service (eg Blogger, Flickr, Facebook), the more likely it is to be stable and experience less downtime than smaller, pioneer applications. They are also less likely to be withdrawn from service or to have substantial changes to their terms and conditions. However, you may find a pioneering service fits your requirements more closely.