Enterprise Overload

I have a query.

facebook is consistently unusable as it is SOOOO slow.

Since it is based on php technology you could suggest that there is a scaling
problem with php.

However this forum can be slow also.

Therefore does anyone agree that over a certain threshold (number of users
making requests) the Web model simply can't cope!

 As there is a cumulative performance hit for Web farms as they increase in size
(my opinion).

 any thoughts?

If this were true the most visited sites such as google, youtube, yahoo, msn
would be incredibly slow.  But they are some of the fastest sites on the web. 
The stateless architecture of the web makes scalability limitless.