I've been looking at the census figures and the beach has lost about 10,000 people since 1980, which is counterintuitive given all the new construction since that time. I mean the Miami Beach of 1980 and the Miami Beach of 2005 is like night and day.

I'm guessing since all the old retirees who died out were permanent residents counted in the census, many of the new residents replacing them are transient and use their housing maybe only a couple months out of the year for vacation and aren't counted in the census figures.

Also, I'm guessing a lot of the sub-standard housing efficiencies (<600 sq. ft apartments) have been gutted and combined to make larger apartments and condos but that creates fewer units.

Anyone have other ideas?

Your first guess is right on the money.
As for the second idea...good thinking, but usually the >600 footers aren't combined.
I believe the MB population is 87,000. There are a hell of a lot of highrise condos that are second/third/fourth homes.
And if the census guy came around at 9am...the old ladies that they used to see on the front porch are now the people that just walked in from the club and they didn't answer the door!!!! :)

87,000 is Miami Beach's year-round population. With the seasonal residents, the island has closer to 135,000 or so.

I think I read once that if you count the permanent residents, transients, visitors, and tourists staying in hotels, that on any given night during the winter season, the Beach can have 200,000 - 300,000 people.

^really?? damn!!

Don't forget creeping affluence. People who can afford $600,000 condos generally don't subdivide them into a half-dozen illegal apartments and rent them out to poor families. And if they try, their neighbors will raise hell at City Hall.

I'd conservatively guess that at least a quarter of the rental units that used to be available 10 years ago have since been eliminated and consolidated into larger units.