I guess once you make enough money you can capitalize however you want. Like Jerry Yang sending all-lowercase emails.

Probably because he's used to type the name of his old company MicroSolutions.

Its original name was "Micro-Soft", IIRC. Maybe Mark has just been around long enough to remember when the capital "S" was correct. (In his defense, he does get the lower-case "b" right in "Facebook", which many people capitalize erroneously.)

Or it's intentional and he's commenting on how they've been too soft in the marketplace these last few years.

Maybe it's an unconscious slip about his MicroSoft member.