Spyke
ccx
sopuli.xyz

I'm probably missing something, but wouldn't it be far easier to redirect people to install page of extension for their respective browser? Such extension could then transform the button as needed to point to whichever social web instance.

2

An extension could work, but some users won't want to install an extension and other users can't install them (work computer/mobile browser).

4

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Building an open share button for the distributed social web | Spyke