mjhjmtu

The second-edition setting Planescape introduced 15 factions for player and NPC usage. I'm in the early stages of running one of the Planescape adventures and I'm wondering: how common is faction membership? Are most people members of factions? Is it more common among certain subsets of the population?

From TSR 2600 Planescape Campaign Setting's A Player's Guide to the Planes, page 14:

Every planar player character must start with a faction, and once a faction is chosen, the cutter is pretty much stuck with it (...)

Prime characters don't automatically start with a faction. If the DM is running a mixed campaign with Sigil as its base, he or she can let prime player charactes begin with membership in a faction. In that case it's assumed the character has been on the planes (as an apprentice or whatever) long enough to sort out the philosophies. If prime characters are adventuring home from a prime-material campaign, they won't belong to one of the factions and will automatically be considered Outsiders. Those folks might have character kits if their DM allows it.

So depending on whether you consider the "Outsiders" and the "Indeps" as factions, then everybody on the planes is on a faction, since the Indeps are folks who think that "...there's nobody who's got a sure key to the truth, so it pays to keep the options open," while the Outsiders are "...adventurers who have wandered in from the Prime Material Plane, folks who aren't part of the planes and don't understand all this business of factions, philosophies, and whatnot. (...) They're not part of the never-ending debate that rages throughout the planes." Otherwise, everybody in Sigil and most of the Outer Planes declares Faction allegiance to one faction or another.

Of course, a few of them won't be forthcoming with the information, and there are actually more factions than just the fifteen (fourteen? thirteen?) — those are simply the largest ones. But the idea is that the Factions are like Planar political parties, and nobody is without one (if they say "I don't want to commit," they're Indeps).

Naturally, membership in particular Factions is more common in certain subsets of the population, while other subsets may preferentially belong to other Factions. For example, the Harmonium is quite popular with law enforcement and tavern bouncers, less so with scholars. They all still belong to one Faction or another, though.

As detailed in the answer by Wtrmute, A Player's Guide to the Planes from the original Planescape Campaign Setting (PS:CS) requires each planar player character to be a member of a faction. Only prime characters who are adventuring home from a prime-material campaign are considered outsiders and not expected to declare factions. This seems to imply that all or most people belong to one or the other faction. Even those without any clear philosophical position can end up in the "Free League" (indeps).

Such an interpretation is reasonable if you are playing a campaign with the basic PS:CS at your disposal. However there are two caveats:

• 
  The PS:CS generally assumes Sigil as the home base for player characters, that is why planar characters are required to start with a faction. However the planes are huge and assuming everybody to be members of Sigil's factions would not be correct. If the factions are likened to political parties, the 15 factions in PSCS are the political parties of Sigil. At other locations throughout the multiverse, beings might have the same parties, but might have completely different parties as well. From the Planes of Chaos boxed set:

Surprising as many Cagers might find it, not everybody on the Great Ring ascribes to one of their factions. For one thing, there are a lot of backwood communities that've never even heard of Sigil. And then there are those folk who have heard of Sigil, but figure it's just some pretentious madhouse that doesn't hold much significance to their lives.

..., in some places, local philosophies and concerns outshine Sigil's factions in importance. Within the githzerai strongholds in Limbo, for instance, the politics of terrain shaping is extremely important. And from Ysgard there has spread a philosophical outlook whose adherents are known as the Ring-givers. Among Cagers, it's common practice to refer to such groups as 'sects', so as not to confuse them with the factions who run the various aspects of Sigil.

• The timeline you are playing might be important. In the past, there used to be many more factions in Sigil. Around year -500 (500 years before the beginning of Factol Haskar's Administration), in an event called Great Upheaval, the Lady of Pain decreed that there can be no more than 15 factions; hence in the PSCS we have 15. However, if you play after year 130, ie. after the event called Faction War, there are no organized factions in Sigil as the Lady of Pain has disbanded them. Of course the banning of a political party does not make its members to disappear, but the power structures changed. Quoting from TSR 2629, Faction War:

The Lady outlaws the factions' power structure, not belief in their ideals. Planars know (and so does the Lady!) that ideas can't be killed. Factioneers don't have to abandon their faith; they just can't rely on having an official organization to protect it anymore.

In conclusion, the Outer Planes are all about belief. That is why the planes are organized around the intangible concept of alignment. Good-evil, law-chaos become physical in the planes. If you see the factions (and sects) as only their core philosophies, you will find some philosophy in everybody's heart in the Outer Planes. However if you are talking about the power organizations around those philosophies, not everybody is required to be part of an organization; though it helps banding together with others sharing your ideals.

Some Actual Statistics

There used to be an NPC list compiled from all official Planescape material, hosted on planewalker.com. Unfortunately it is no longer downloadable, so I searched on the web to find an old copy at this URL. It contains 742 entries, but let us focus on the 107 that have 4 or 5 stars, meaning a good amount of data is available for the DM to flesh out the NPC, including most of their game statistics.

Out of 107 fully-fleshed out Planescape NPCs, the alignment is listed for 104, while the faction/sect is given only for 78. This, of course, does not mean that 75% of PS characters are a member of a faction or sect, but it gives us an extra clue about your question. Please note that the list is likely to be biased towards having a large number of characters from Sigil, so take into account the caveats highlighted above.