I think Twitter has just about reached the market saturation where first adopters will start looking for something new. It's considered a legitimate news source and most social networking users are hip to it, if not our parents yet. With Twitter becoming mainstream what could be next?
Posterous,Tumblr, and similar services have been slowly gaining popularity for a while. Situated somewhere between Twitter and a blog, these are ideal places to upload brief clips of text, day-to-day snapped photos, and the occasional short video. While they're a brilliant amalgamation of current web communication they seem to be a bit too involved for the average webizen.
Robo.to is a service between Posterous and Twitter. It's in my opinion a simple step above Twitter, adding Facebook and blog integration and a short, 30-sec looping video of yourself. While enjoyable I think the reason it hasn't taken off has been the webcam feature. Yes it's incredibly fun and an innovative integration but as curious tech users have found since the debut of webcams we aren't always looking our best when surfing the web. As much as I love Robo.to I rarely update it because most days I'm sitting in front of the computer in frumpy pajamas and with wild, unruly bedhair.
What these services and most speculation don't take into account however is that the next "hot" webapp is bound to be something we can't quite imagine just yet. As much as Facebook is going after Twitter because they believe them to be competition, they really aren't. True, the service is taking FB's place in the internet spotlight, but it's proven itself more of a news service than a social networking platform. The next big thing may bear some passing resemblance to the two giants, or may take one of their features and expand upon it to the point of a worthwhile, brilliant service in its own right, but it will be an unlikely service we did not instantly recognize for its similar potential.
Taking that into account, what are the basic characteristics of both Facebook and Twitter that it may have? Both services let users share simple, small content broadly and include a networking component. While we may not recognize it's form, it's a good bet the next web service hit will fit these basic criteria.