The accepted answer, asserting that WiFi is not a noun, is incorrect. WiFi is clearly a noun. It is probably used mostly as an uncountable noun. We are unlikely to say "I have two WiFis," just as we wouldn't say "I have two furnitures."
However, as choster's answer suggests, some may be using it as a countable noun. A google search for the exact phrase
"there are two wifis available" yields one result with a snippet from a tripadvisor review saying
there are two wifis available, pick the right one -depending on where you are and you can surf no probs unlike some of the reviewers who experienced trouble.
It's probably good style, for formal writing, to avoid this, but the question asks "do people pluralize 'WiFi' with an 's'"; the answer is apparently "yes." If the question were "should people..." then the answer might be "no."