A very interesting article. Below are the pros and cons of each service. Take a look.
Twitter Pros
* Easy to navigate and update, link to and promote anything
* Reach far beyond your inner circle of friends
* One feed pools all users; anyone can follow anyone else unless blocked
* Pure communication tool, rapid responsiveness
* You dont have to be logged in to get updates; you can just use an RSS reader
* Very interactive, extensible messaging platform with open APIs
* Many other applications being developed (Twitterific, Summize, Twhirl, etc.)
* Potential SMS text messaging revenue from wireless networks (although Twitter states they are not currently getting any cut)
* Potential future advertising and/or enterprise subscription-based revenue streams\* With its thin overhead, Twitter is probably more scalable than Facebook, giving it a cost advantageTwitter Cons
* Limited functionality; find people, send brief messages, direct replies
* Limited to 140 characters per update
* Not all people find it immediately useful
* Over-emphasis on follower counts
* Easily abused for spam and increasing the noise level
* Relatively smaller installed user base
* As yet no readily apparent monetization strategy

Facebook Pros
* Application mashup; find people, make connections, email, instant messaging, image/video sharing, etc.
* Most people can quickly grasp the value of connecting with friends, family and established contacts; some people report they use Facebook instead of email and IM
* More emphasis on deep connections with others vs. who has the most connections
* True Friends feature increases your transparency to selected connections; almost like having private and public profiles
* Huge, rapidly growing installed user base
* Inherit stickiness, third party applications, gift giving and personal data collection make Facebook a powerful advertising platform
Facebook Cons
* More difficult to navigate and update
* Requires investment of time to realize sustained benefit
* Opt in model requires a user to allow others to connect
* Less immediate responses; unless you stay logged on continually
* Overhead of mashup and thick applications could limit scalability, bloat cost structure
via Facebook vs Twitter.