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Other than just being design choices made by different companies (and the trademarks/copyrights that come with them) you must take into account what the icons are intended to represent. A good icon should denote its meaning without any supplemental text (although you should still have it). An icon that denotes an action such as "share" should represent that action using its perceived motion. Without looking into what each icon there actually stands for here is what I as a "new user" would think they would represent:
- Exporting (looks as if the data is leaving the client such as to a server)
- One-to-Many sharing (One node is sharing the data with others in a one-way flow)
- One-to-Many sharing
- Peer-to-Peer sharing (sharing is one to one but in a symbiotic full circle flow)
- Gifting (implies the object has a value as opposed to just a file, stimulates emotion)
- Not sure, perhaps that the data is more stationary and the users come and go to it such as a git repository?
So as you can see different icons represent (at least to me) different things and thus are used in different places, pick the one that fits your situation the closest.
We could go on and on for hours throwing conjectures and theories, but in the end, it's only a branding thing.
Your icons , from left to right are from Apple, Apple, ShareThis, Android, Windows, Windows and the Open Share Project. Exception made of the last one, they all belong to companies that won't give up on their efforts to brand their apps as they want, hopefully establishing the standard.
Now, if you want to know which one is the most popular and "safe", then the ShareThis one is the most popular and easy to distinguish by users*, which has a very easy to explain logic: while a bit cryptic, it has been used in so many sites through the years that anyone who surfed the web for at least a few months has been exposed to it thousands of times, while all the other icons are "device based" so people not having the specific device won't understand the symbolism.
As an example, in our research we noticed a bias towards your first icon on Mac users, while most Windows and Linux users thought it was an upload icon. Normalized data shown that the only icon everybody recognized as a "share" icon was the "3 dots" by ShareThis. However, the open source community is trying to give more exposure to the Open Share icon, so maybe this will change in a not so distant future
- (note: based on our own research, which didn't include the last 3 in your list by the time we did it, but I'm sure many people here did the same research since it's an obvious thing to do, so expect more results that might or might not be like ours)
According to this article, a standard well accepted share icon does not exist :
It is unlikely that we will see a convergence to a single share symbol. Apple will not start using Android's design language, Google is not going to implement Microsoft's design, nor is Microsoft going to use another platform's share icons. Since each of the big three OS companies has huge device market share, users will likely interact with at least three different types of symbols that represent the same action.
If you want to have an icon that is understood by almost everybody use one of the following options :
- Use a symbol with a label. I find the Y symbol the most intuitive and the most wide spread. Add to that symbol the label "Share"
- Create a group of icons with the bigest networks, and then add a three dots icon for the rest of the networks :
- Use only the word "Share". Make it blue and underlined so that users will know that label is clickable.
According to the comments a button is more appropriate than the link style above, which I tend to agree :