I know a lot about the places I visit, and I want to share it with everyone.
I have been donating and volunteering at OpenStreetMap, and I am also aware of Google Maps.
Where else can I help with collaborative mapping?
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Of course it is ultimately up to you, but i would hesitate to call working with OpenStreetMap a waste of time.
While it is true that the OSM user base is small right now, there are factors that affect both Google and OSM. The primary issue is that now that Google has started to charge for levels of use of their API, a number of companies and organizations are switching to OSM. The list includes Apple, Foursquare, and Wikipedia mobile apps. It looks like more may switch as well, which could increase the user base significantly.
This is one aspect.
The other, and that which seems more important to me is the idea of open access to data. As a Geographer and GIS professional, having access to high quality datasets is critical for me to do my job. Having access that can not be compromised is even more important. The OSM dataset is licensed as Open Data Commons Open Database License, so anyone may use it. Google data is privately owned and while this likely won't happen, they could close it off tomorrow and charge everyone for access, whether you helped improve the dataset or not.
I know that here in the United States access to data is a big issue. A lot of time and money is spent collecting and building the datasets, and a big question is who should pick up this cost. There are arguments either way. I am sure this is an issue in other parts of the world as well, especially in places where there are not as many sources of data like streets available for people to use.
This is a pretty hot topic you picked here, with strong, valid feelings on both sides. Google has every right to charge for their data, and make a profit. People have the same right to build their own dataset and make it available to everyone. There is nothing to say you can't support both, just don't sell one short over the other without researching on your own, and drawing your own conclusions.
It's worth noting that at Google Maps, you're not 'donating' your geographic knowledge -- you are giving your geographic knowledge to a for-profit corporate entity, who have a duty to their shareholders to use their position as a majority data holder in the worldwide map data market to increase the value of their company.
This means that the data that you give to Google is not necessarily something that you can get back out and do what you want with. For example, if you imagine that you start a business making GPS Maps for Bikes, or some such, down the road, getting the data that you give to Google back out for use in the product you want to sell may be difficult, or simply a violation of the Terms of Service which you would not be allowed to pursue.
OpenStreetMap is openly licensed dataset. It is created and supported by more than 500,000 editors, and the data is available for libre (that is, 'free as in freedom') use under the terms of the ODBL, which would allow you to make the GPS Maps For Bikes app without needing to pay a licensing fee to anyone.
Data you add to Google Maps is collectively licensed to Google. (Your individual contributions presumably have relatively limited value outside the collection of data, as is the case in almost any map-making effort.) If you do something Google doesn't want, then they can say No, and shut you down. Innovation by third parties outside the scope of what limitations Google provides for is not allowed.
OpenStreetMap data is openly licensed and managed by a non-profit organization dedicated to furthering the use of geographic information around the world. If no one has done the new cool thing that you want to do -- you can get the data, and do it.
Short term, you may get more out of Google -- with a massive corporate investment in user-targeted tools, and a massive investment in initial data -- but over time, OpenStreetMap's openly licensed data is likely to be the more sustainable solution. With half a million editors, and the growing investment from national mapping agencies around the globe, I can't imagine any way in which OSM is not the more appropriate map provider to support for the long term.
In addition to the other excellent comments here, the biggest advantage I see to OSM data is two aspects of ease of access:
(1) in particular has been annoying recently. I have battled with Google editors for approval of known bad data, or not had my GPS-derived roads approved because it doesn't match with the 8-year old satellite data!
Of course, the ease of access is only really with us because of the small base of users. The more open it gets, the more problems we'll have with validation and vandalism.
Go with Google. They currently provide a very good free service to everyone in the world for free. It is always being developed by a very large team and the data is very accessible and usable through the Google set of products which millions use.
The mapping architecture and software developed by Google values the user 1st most and Google only makes money from users through the showing of advertisements, not directly charging the user.
Selling API access to their database is reasonable, and only large websites ever have that issue, but who cares? If you're not running a website then it doesn't matter to you at all!