Approximately 2500 prims were used for the huge Leviathan skeleton on the new Ahab's Haunt sim. Of those 2500 prims, quite literally only a handful are sculpts. All the other prims are regular prims. Not a single prim I found is larger than 10x10x10 meters or - in other words - this humongous build was constructed without a single megaprim.
The skeleton was built by "moles" - content creators contracted by Linden Lab. It is a safe assumption that the moles know what megaprims are, and how to use them. At the same time finding a Linden build (or Linden contracted build) that uses megaprims is like searching a needle in a haystack.
I am not a pro builder, but my guess is that the leviathan skeleton could probably be built with 75% less prims if megaprims could have been used. And probably with 90% less prims if sculpted megaprims would have been an option. This means the asset browser, the SL databases and the viewers of everybody coming close to that island would only need to store, transmit and render about 250-350 prims, instead of 2500.
To understand the (I assume deliberate) decision to not use megaprims, we need to take a step back. Megaprims were never regularly available in Second Life. Initially, the SL viewer limited the maximum size of a prim, and some clever person managed to circumvent this restriction and injected the first batch of megaprims which then got handed down from builder to builder. As a result, the check for the prim dimension was included in the server side, and megaprims could not be generated anymore. A while ago, a bug in the server code made the creation of megaprims possible again, and the second wave of prims became available. At the time of this writing, there are about 36,000 megaprims known and available in SL.
There is all kinds of rumours regarding megaprims. They allegedly cause lag. They allegedly steal sim resources. They allegedly influence the physics model badly. They allegedly are illegal and can get you banned using them. Indeed, the status of megaprims is a fuzzy one. Linden Lab never outright forbid their use. Neither did they take technical steps to restrict the usage. But also they never gave the "green light" for their usage. Megaprims are tolerated, but not loved. And because of this, Linden Lab will be careful not to set a precedent and use these prims in their own builds.
Update February 8:
[11:24] Silent Mole: It's true! Mole builds don't use megaprims.
[11:26] Peter Stindberg: Thanks! Can I quote this?
[11:27] Silent Mole: Yep.
[11:29] Silent Mole: Andrew Linden is the company expert on megaprims. His office hours are at Denby (213/45/34) 11:00 - 12:00 Tuesdays and 16:00 - 17:00 Fridays.