I read this morning that Twitter just secured a $250m VC infusion of operating funds. Good news for Twitter management and users (my handle on Twitter is mark_l_watson) but they do need a revenue model. Facebook (my handle there is my gmail.com address mark.watson) at least runs adds.
I am not very active on either Twitter or Facebook but they are fun. As a consumer, I would not mind at all paying a low fee (like $5/year) for no-advertisement service and to keep them in business long term. As a developer I like to see open APIs with free low-volume use of the APIs and modest costs as use scales up.
The issue is how to get people to start paying for services that they have been enjoying for free. Flickr (mark_watson) got me to upgrade to a paid account when I hit the 200 photograph limit for free accounts. I use Google gmail, documents, and calendar as part of both my business work-flow and my personal life-flow but I have yet to upgrade to a paid $50/year account. I think that Google needs something less expensive for individuals who would still like to pay for some form of guaranteed quality service. (That said, I am sure that I buy enough stuff via Gmail advertisements that they are making money from my use of their services.) BTW, if you base business on third party web applications, make sure you back up locally! (Google makes this easy with their gdata web service APIs.)
Since I am writing a book on Web 3.0 I have been increasingly looking at the web as an application platform and I look forward to more services like Twitter and Facebook (but more focused on many separate market niches) that are both financially solvent long-term and with open (and inexpensive to use) APIs.
The issue of up-time and long term viability is huge for developers building services by aggregation of other services. I was writing example programs yesterday (for my new book) demonstrating clients using SPARQL to query public RDF repositories and was disappointed at how many SPARQL endpoints were "down", at least temporarily. Any Web 3.0 type applications that aggregate information and services must obviously be resilient to unavailable services, and that is just part of the challenge. I think that the key thing is to have not only a technology infrastructure but also a business infrastructure for Web 3.0 applications.