I personally have no idea about typography, but I use double space after a period for semantics --- as a sentence terminator. because, you know, there are languages where periods are used not only for terminating sentences.

as a side note --- capitalising the first letter of a sentence is a convention similar to putting two spaces between sentences. why does the author do this?

Because it's a software update? I don't know what qualifies to be posted on HN, but people post software releases and updates here all the time.

1 millimetre? Who measures Searches Per Day in millimetres?

I read it the same.

M = 1'000 in Roman. So MM is a million.

As noted in another comment; MM is 2000 in roman numerals.

Never heard of such a notation before. Probably only used in the US where people don't use mm.

MM is a million. mm is a millimetre.

In the metric system,

1 mm = 0.001 m = 0.001 metres

but

1 MM = 1000000 M = 1000000 ?????

MM, correct or not, is finance lingo for million (1000*1000).

Just like 1 KB is 1024 bytes, not 1000. ;)