- I think it should feel more like a game than a social network. More like Pokemon Go than Facebook. I'd gamify as much as possible.
- Some people want to plant trees and are able to do so. Others aren't. Maybe they don't have the time, or they live in a concrete jungle. I think the project should focus on both segments.
- Those who can't plant, whatever the reason, may want to donate to projects that would plant. And those who can plant, may need guidance (where to buy seeds? where to plant? what to plant? etc). This website should facilitate both cases, as much as possible.
- Let's say that for each dollar donated, one tree is planted. That number (trees "remotely" planted by the person) should be made public, as some sort of virtual currency.
- Ideally, trees planted as proxy (via donation) would be photographed and shown to the donor. Unfortunately that's not practical.
- I think each person should have their own profile (with their own name, picture) and they could create a "sub-profile" for each tree. Not sure about this. But having a separate profile (login) per tree would actually be a problem for those planting lots of trees.
- (Or, using a better analogy for "sub-profile", people = Github profile, trees = Github repositories)
- This "sub-profile" could be similar to a Facebook group. You can post "life events" of the tree, photos, comments, etc. You can browse other people's trees (and their own, actual profile as well).
- Each tree planted by the users may have a location at the world map (provided by the user). This way we can show nearby trees and fellow "tree planters".
- We can have mini-games like "visit/photograph/'collect' trees near you". Similar (to a certain point) to Pokemon Go. When completing such mini-games the person is rewarded with the virtual currency.
- People may use the virtual currency with their own trees or others' trees. Not sure actually what they could spend it on.
- I really like the idea on sister comment from @secfirstmd. I think integrating related concepts further improves the platform.
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Also, something important to remember, is that this website alone likely wouldn't get anywhere near OP's objective of 1 trillion trees, but it certainly can make an impact on awareness, or at the very least increase people's contact with nature (even in large cities).