There has never been a better time to build your systems on the internet. System level platforms and app market places or eco-systems are being assembled with blinding speed. The Apple IOS app store just announced their 40 billionth download; The SalesForce app exchange market place has 75,000 partners, etc., etc.
Make or Buy, does what you need exist? In the early days of the internet, we had to build these systems from scratch and do the hard middleware work to connect our internal systems to the web. Today, these Software as a Service (SaaS) platforms are investing thousands of engineers to do this heavy lifting for you. The task is in customizing their platforms into the agility platforms that your customers need and want.
In the past, I would always advocate for internal proprietary development so we could create systems that were very hard to duplicate. Now, no company will have the time to do this. All of the separate silos of the internet are being joined through these platform strategies for the sake of speed. In the future, the cycle will continue and open standards will emerge so you can cost reduce these platforms but for now, you are likely to have to go with a platform strategy for speed and competitive survival.