As long as the 53,651 are actually interested in using it, not just having a look or thinking about the startup behind it. I imagine thats the problem with traffic from places like techcrunch.

I agree, but it seems you need to be trying to cross that chasm. I was just reading http://www.height1percent.com/articles/2006/08/18/actual-lessons-from-kiko (near the bottom of the main entry- "You must have a plan for escaping the Technosphere" )

I wonder if user numbers are quantized at the Early Adopter number, or if successful sites smoothly cross that threshold (like the paid user quantization). For instance, did myspace or youtube have a user population stall around the 50k mark before becoming more widely successful?

Yes - if you can "cross the chasm" at some later date.