It is clear from all usage reports that Twitter is definitely used by more people than the other services, but I am still not sure whether any of them are truly must have or useful. Rejaw and Jaiku are more in line with the Twitter model, but they allow users to make comments on posts. Jaiku goes further by allowing channels, which operate much like an IRC Channel. You follow the channel and when users post updates to it they are displayed in your feed. Tumblr is a very nifty little blogging application that allows you to post media and text fast. This is great for sharing an image or video you found on the web that you liked.
So that is very interesting, but what in the world is the point? I am still yet to work that out myself. Why follow someone on Twitter when you can just as easily subscribe to their blogs RSS feed on any number of sites such as Bloglines or Google. A lot of people are only using Twitter to let their followers know that they have released a new article on their blog for which RSS feeds are just as useable. Both push your article onto the top of your readers list.
You may argue that Twitter et al allows people to respond about articles and push them to their friends by reposting the link to their own feed, but this can just as easily be achieved with links to your blog from their site and by using the comments for on your site.
Much has been made of the use of Twitter during the Mumbai terrorist attacks, but I am unsure of the usefulness even for this purpose. None of what is being posted has been peer reviewed or fact checked as newspaper reports are supposed to be. You may be getting the most up to date reports, but what of their accuracy? I will stick to my BBC and Reuters RSS feeds I think.
I feel Jaiku is the better of all the services mentioned but because you must have an invite to sign up less people are using it and therefore it less engaging. Also I feel the channels feature is redundant as people are already using forums, mailing lists (Google Groups being a good example) and IRC for the same results. It does allow you to participate in the conversation away from your computer via MMS and WAP plus it also has an S60 compatible client, which is difficult to do on the more traditional communication mediums.
Even as I write this I am struggling to think of ways that any of these services might be useful. This is also partly due to the fact that all my friends are on Facebook so I can use that to send them presence/status updates and let them know what I am up to. What does anyone else on the web need to know about my latest thoughts on fried chicken or what magazine I am thinking of buying? Maybe your friends on the other side of the world might like to know so they feel they are keeping touch, but outsiders want considered thoughts and well written to the point articles not disorganised dross.