On another question I commented that I doubted that if Lojban had a functional speech community that its purity would last very long.
By what criteria can we say that a conlang has a functional speech community?
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Here are some ideas, which when taken together would give a good idea whether a language has a strong functional speech community.
I'd lower the criteria significantly and already admit that a conlang has a speech community when it is used on some occasions for real-time face-to-face communications. By these criteria, even Klingon has a speech community.
Whether a speech community is functional or not is probably difficult to decide, I'd go for a kind of temporal criterion like having a speech community (as sketched above) for 30 years with no year left out. This is similar to the criterion biologists use to define an established new species in a certain area.