I have Twitter friends that post a few times a week, I have Twitter friends that post every 5 minutes, I have Twitter friends who, 99% of the time, are in reply-to conversation with people I don't know, so their tweets are meaningless to me. I have yet to see the killer app for managing that aspect of the service. Assume there's an audience that mostly uses Twitter for reading, rather than just for hearing themselves talk, and helps sort out what they're listening to.

There are so many twitter apps that my head spins. Create a website that teaches you how to use twitter efficiently. Start from scratch, presuming that the user has no idea how in the world would twitter help him in his daily life. Present the most useful twitter apps with screenshots and tell him how could he use them in an efficient way. Create tutorials on how he could integrate twitter into other sites (with the help of wordpress plugins and such). Target different categories of people and give them personalized advice. Show unusual uses of twitter, marketing and advertising tactics, and even how can they make money out of it.

1. For facebook; you can create a chess game application, so friends invite each other for a friendly game of chess.
2.Create application so people with same interests can share their books.
- you can call a "Book Club"
3.Something like a Weight Watchers progress bar.
4.Create backgrounds.
5. Also it would be nice to have something like "Cash Cab" quizzes for common knowledge trivia questions.

I hope this helps a little :)

There are way too many Twitter apps, as mentioned above. Make a site that explains these apps, ranks and rates the apps, helps people understand how to use them, and allows them to use all the apps in one place without having to go from site to site. You can call it TwitterFor.com and it could be a site about "What people Twitter For," then interview people and ask them "What do you Twitter for?"

If its a website that you are interested in let me give you ideas on the kind of platforms you should consider this coming from a professional web applications developer.

If you want to create an e-commerce website I strongly recommend you to use magentocommerce it is a Free and Open Source Software Platform that was voted the best e-commerce platform for the year 2008. I have been using it for a while and my client's love it,and the beauty is that it integrates seamlessly with all major Social Networking Web 2.0 widgets/ Gadgets ,so you can incorporate all the Opensocial Apps.

If on the other hand you are interested in a social networking platform or website my recommendation t you would be to use elgg v1.2. I am currently in the process of creating a Social Networking Website using this platform which was also voted as the best such platform during last years Best of Open Source Software ,BOSS web awards.It also is opensocial meaning you can incorporate all the apps you can think off to suit your particular needs,from Google code gadgets to opensocial widgets.

Both these platforms magentocommerce and elgg are freely licensed meaning their source code is available for modification as long as you follow some creative commons attribution,if you are interested in any of the above

I would find a Facebook app that allows me to easily see and go to new posts in discussion threads very useful.

The main screen would show something like:

Group X:
Thread ABC (5 new posts)
Thread DEF (2 new posts)

Group Y:
Thread GHI (4 new posts)

Optionally you might show the first line or two of the posts as well.

Then if I click on one of the thread I go straight to the first of the new posts in that thread, and can scroll through all the rest of the new posts.

Also there should be some kind of "Mark as read" facility where I don't have to actually read the stuff to clear it out, and reset the count to zero.

i.e. Add some basic forum type functionality to the discussion boards within Facebook groups.

As far as I know, such functionality doesn't exist. But if does, please tell me where to find it!