As Cerberus stated, "Factions" is any organization in the game. With that in mind - allow me to provide my perspective on what we aspire to accomplish with any faction while addressing the feedback provided so far.
Visibility Every faction has a corporate culture...even if they're less than corporate. Visibility largely plays into it. A corporation may have jobs on payroll entirely dedicated to creating more visibility, a gang may cover the streets with graffiti along with shouts over SIC quietly, and more clandestine organization may go to great lengths to remain unknown. With that said, one of the things which makes Sindome so great is the flow of paydata. It is the jazz which keeps chyen flowing and people talking. Without obfuscation there is no paydata.
Accessibility Some things in Sindome are particularly challenging for a reason. A corporate gig is fairly barebones. You work a job. You collect a wage. The price of entry is kept themely but not painstakingly high. The nice thing is that due to the high number of PC's involved - I like to imagine corporations are a bit self-governing.
A ganger on other hand typically has less member and very different stakes. There's no wage so PC's are responsible for making there own biz. They're in the limelight constantly exposed to NPC and PC threat from rivals. It's a tough gig and while people occasionally try to join the ranks - it is difficult to survive long enough in the role to rise to notoriety. I think this is why you see less PC's throughout the ganger population. It a totally valid "career" choice though.
As for other, less seen factions. I believe they are difficult to join because they whole idea of being behind the scenes means that many PC's do your work but few ever really knows who they're working for.
In essence though, I think accessibility is pretty consistent across all organization. The easier and more low end the gig - the more accessible it is. However, depending on the organization the jobs are predominantly high level. The WJF is a good corporate example of this.
Focus and Flavor This is one area where I think PC's have the most influence. As players, we really dictate the focus and flavor of the organization we are affiliated with. NPC's are there to reinforce it and keep things from sliding off the rails. You may find an organization culture still being driven by ideas and concept from a long dead PC in some cases.
Balance Not all factions are equal. However, most factions have rival factions which put them on a roughly even playing field. Some factions just have better PR in some cases. Another thing to remember though is that the struggle is real in SD. An underdog who can't confront there rival directly - can aspire to bigger and badder things. The game encourages dynamic actions to fight your foes so don't spend too much time trying to measure an organizations power. Also, see my prior comment about visibility. ;-)
To ExMachinae's point, there's a lot of low level opportunities which could use some PC love.
Let's keep this going.