"Indispensably useful" has positive connotations, yet most people I speak to (even outside the HN/tech crowd) do not speak of Facebook in positive terms.

Some use it simply because it's addictive. They don't want to use it, they don't enjoy using it, but they're eventually drawn back in after trying to quit. In this sense, it's "indispensably useful" in the way crack cocaine is indispensably useful.