Note that we didn't run stock freebsd. It was a custom freebsd4 with a custom gcc 2.95 toolchain. I don't remember all the mods, but migrating from 4 to 6 was a nice undertaking, and it was done in parts. People who worked there will remember the term "4 on 6". After the migration, the kernel was 6 (and you got the latest drivers) but we were still running those weird 4 user-land binaries.

Inktomi (YST) was acquired in 2003, later Overture (YSM) and both were linux shops. YST was the biggest property by far and we didn't have any kernel developers. We were doing just fine with stock debian, then stock RHEL. (I did a few very minor patches for 32-bit, like splitting user-space/kernel to 3.5/0.5G instead of 3/1G, and some caching improvements for /proc, but I wasn't a kernel developer). Linux just worked. Drivers were available and supported for the HW we needed. Oprofile, systemtap worked fine for the most part. We later hired one kernel developer to help Mail move to linux, but we always had more freebsd kernel developers.

For freebsd you needed to write the drivers and the tools. That took time and money. The community was also smaller and it was harder to hire people with experience. Acquisitions were running on linux too.

The decision was not easy. Y! already had great freebsd engineers. By announcing that linux would be the supported platform going forward it was a given that many of them wouldn't be happy and leave. Fortunately not all of them left. For example Peter W. still works there.

In any case I don't think there was really an alternative.

Even when Rick was at Y! the push to move to linux was there. Rick had written most of the base libraries for freebsd originally (mdbm, ylock, ylock_kern, yfor, etc.), and tools like the package manager yinst and ysar, but being the great engineer he is, he ported them to linux and got the same or better performance. (In the yinst case he delegated the maintenance to a very capable engineer)

Note that Yahoo! was employing a few very bright freebsd kernel engineers but when the decision was announced that linux was the future many of them decided to leave. That made the path forward even clearer. It wasn't just a matter of integrating new acquisitions easier, and the fact that it was easier to hire people with linux experience than with freebsd experience. That was around 2008 IIRC. I left in 2010, and Rick less than a year later.