I'm writing a detailed note on Facebook Home for Enders Analysis, but some of the distribution maths struck me as worth posting here as well.
Facebook Home will be available in the Google Play app store from April 12, but at launch it will only be supported on a limited number of high-end phones (HTC One X, HTC One X+, Samsung Galaxy S III and Samsung Galaxy Note II).
These devices together have sold perhaps 60-70m units (we know 40m GS3s a month or two ago but the rest is pretty speculative), out of a total of around 680m Androids in the last two years. The GS4 will presumably be supported too, of course. In addition, HTC is launching a new mid-range Android phone (the ‘First’), which will come with Home pre-installed. Sadly, given HTC's current position, I don't expect this to change the trajectory of anything.
(Incidentally, it's a sign of how banal Android fragmentation has become that the fact Facebook has to give a list of supported devices has passed largely without comment.)
This is just a first step: I'd expect Home to expand to cover most or all devices running Android 4 or later in the next six months or so, if not sooner. In March Android 4.x made up 54% of the active Android base outside China, according to Google’s developer statistics. This is moving up, and will be maybe 75% by the end of the year. Android 2.3 is possible, but a lot more work and seems unlikely.
Expanding Home to cover all 4.x devices might take the addressable base to 375m.
Meanwhile Home is not available for the iPhone and almost certainly never will be, since Apple would not permit such a take-over of the interface.
In December 2012 iPhone users made up 29% of monthly active users (MAUs) of Facebook’s smartphones apps; Android was 38%, and growing faster (it was about 35% last summer). Facebook stopped disclosing this data at the end of the year so I don't have more up-to-date numbers.
So, all of these numbers are moving, and some are a little fuzzy, but it looks like Facebook Home might be available for something like 20-25% of the current base of Facebook smartphone apps users today (assuming it really does expand to cover Android 4.x). By the end of the year, Android might be 45-50% of Facebook's base and have 75% Android 4.x penetration, which would take that to maybe 40%.
How many people actually will install it (and keep it) is another matter entirely, of course.
As an aside, I always prefer to talk about workings and variables than just state 'it will be x'. Instead of the above, I could just say "130m people can use Home", but that approach always seems less helpful - if not rather arrogant. After all, no-one at all actually knows the real number.