The original Orkut site was written in C# - it was very slow and couldn't handle the growth. Orkut was only a big success in countries like Brazil and India where internet connections were already so poor that nobody noticed how slow the site was.
These are orthogonal facts presented as if they were related.
That doesn't mean C#'s execution speed was the primary bottleneck. In fact, Jeske's team reportedly made dramatic performance improvements while retaining .NET. But arguing that the choice of technology had no relationship to Orkut's scalability problems is just as misleading as blaming everything on C#.
If my current employer started having new software written in C instead of Java today, and while C has many problems, the problems we'd have would be all self-inflicted skill issues, not a direct problem with C.
If they had opted for C# they could probably have been able to run it as-is today.
That doesn't mean PHP prevented Facebook from scaling, just as C# didn't inherently prevent Orkut from scaling. But being able to scale an application and being able to scale it efficiently are different things. You can compensate for language and runtime inefficiencies with more hardware and better architecture, but that doesn't make those inefficiencies irrelevant.
C# would arguably have given Facebook a better starting point in terms of execution performance and static typing than PHP, but claiming they could have run the original application unchanged today is a bit of a stretch. Facebook's architecture, infrastructure and requirements have evolved enormously, regardless of language.