I'm going to make some arbitrary assumptions based on certain things before I start working on my answer just to clear things up:
I think that it's not unreasonable to say that 50% of mages who are professional, fully-trained mages, have taken at least one initiation. In addition, 70% of initiated mages have probably joined a group.
If we assume that the groups listed in Street Magic make up about 10% of the mages that have initiated that belong to groups, we have 1203 mages forming 3.5% of the population of mages who are "professional" full-time wage mages or the like. This gives an estimate of 34,000 mages, which doesn't necessarily include latent magicians or magicians who are not "dedicated" mages (i.e. people with a Magic rating of 1 or 2, burnouts, and possibly a decent chunk of adepts), so you've got a fair amount of people there that either have some powers but don't really function in society as mages, and those who have very little magical power and aren't likely to fling a spell or summon a spirit, which would be the majority of people who are capable of theoretically using magic.
This number seems a lot more realistic to me, when we consider that Lone Star, for instance, has a magical investigation group in Seattle, which is almost certainly more than six people. In addition, stuff like tempo has started to blur the line between Awakened and not, and the openness of arcana to non-mages means that the magical is potentially much more prevalent than its users.
As far as the people most likely to use magic; other than some metavariants they're mostly pretty evenly spread throughout society, being a great equalizer-the punk in the ghetto is just as likely to have mojo as a corporate head, though we can expect magic users to climb up economic classifications on account of their power compared to someone without magic.
See my Notes! below for a preface!
This is a technique based in misdirection, so most of the examples I've used/suggested/come across follow suit:
1) You hacked into a system to steal or erase a file, and you want to leave as much time as possible so you can dump it or escape before it is noticed. Unfortunately, someone is aware you were having around in the files. You could use Resonance Veil to make them think you were after another file.
2) That security spider hadn't seen you, yet, and you want to make it and it's perception dice go looking elsewhere. Make it think that it saw an intruder somewhere else.
3) Drek, you didn't send the spider off before and now it's noticing that a feed looks different than how it should. Time to make that spider think that the camera is glitching and needs a rest.
4) A runner was trying to incite gang A to attack gang B. Both gangs were pretty level-headed, and though they were rival gangs didn't want to attack each other without a good cause to call the other gangs to. Dropping AR tags of each other's colors wasn't working, so instead they waited until gang A's patrol was around and dropped a tag in front of them, using Resonance Veil to make it seem like it came from gang B. Instant war!
** Notes! **
I'm putting this down here because it's not part of the answer, but might help in understanding the problem that brought forth the question.
First, I feel like I need to address Technomancers: Until Data Trails comes out (soon!), Technomancers have little to go on aside from what's in RAW. A sharp mind can make a basic technomancer that breaks things easily, but fortunately as a GM there are plenty of ways around that. That's beyond the scope of this question.
Second, can this skill be called useless? Sure. It doesn't really have a direct mechanical impact. You can't use it to directly help you with anything, which is why it's a skill better suited for the more covert data wizards.
Third, is this skill broken? Mmmmm....mostly. No more broken than Puppeteer is. And all of these things can be taxing on a technomancer. To make a convincing illusion can put a decent amount of hurt on a technomancer's brain. If you're going against a security spider who has a decent Intuition and Data Processing skill, you're going to want to try a Force 6 to get some net hits. That's five fading you have to resist, and hopefully it will work. If it does work and you hit net hits they now have make a Matrix Perception Test (assuming they feel the need to make one)
Ultimately it's up to you and your GM to make sure it doesn't get too powerful, but if you ask me personally it's just fine. There's a decent amount of payment in the form of fading for the benefit, and the benefit isn't something that will keep your character from dying.
Best Answer
I never saw something like an official answer, but the novels always gave me the impression that most of the middle class would have a datajack and a decent percentage would have a chipjack. Cosmetic surgery would also be fairly common.
Most of the other enhancements were probably much more rare. The rest were all the type of things that you would shell out for if you had a need (either to make up for some sort of physical defect, or for your occupation), but probably wouldn't get without a real need even if you could afford it.
As you point out, the upperclass can easily buy enhanced muscles, but why? Enhanced muscles means going through a major surgery as well as the monetary outlay, and that would largely enable you to do things that you would rather hire people for.