Joe --
It's important to keep software up-to-date and patched, and OpenSim is no exception. OpenSim hosting providers need to keep up with the latest releases of OpenSim and install patches when necessary. Sometimes updating the software requires migrating databases.
Bandwidth is how many simultaneous visitors you can get on your site or grid. If you're running a grid for free, on a home computer and with a typical home Internet connection, you can probably get five people in, if there's not too much else going on. On a professionally hosted region, you can get between 20 and 100 people, depending on your hosting plan, and several hundred people on a region if your vendor is running DSG.
I'd say that there are a handful of grids and hosting vendors now that have full-time management or employees.
As an estimate, say that you're renting regions for $50 each, and the hosting costs you $25 each. That leaves you $25 per region to pay for staff costs. A grid with 100 regions thus has $2,500 a month to pay for a manager or developer.
I count about 15 commercial grids with more than 100 regions, plus there are the independent hosting vendors -- who do not release their customer numbers except for one that told me that they're running "hundreds" of private grids. Some grids charge more than $50 per region, and I'm sure the larger grids try to buy in bulk, and can get their server and bandwidth costs to below $25 a region.
Then you have to add in the value added vendors -- consultants and designers like Rivers Run Red that use OpenSim to build simulations for large corporate clients.
It's still a small number of people overall -- maybe two or three dozen altogether -- who make a living from OpenSim, and some of those work in other platforms as well, such as Unity.
That's up from zero three or four years ago, but you're right, it's still a very slow pace of growth.