I don't think it's plausible that the police will routinely rely on arrests to get social media information from random stops, for a bunch of reasons; part of it is that it's unlawful for them to do so, but the bigger part of it is that it's incredibly inefficient and painful for them to do that too.
If the argument is that the police will use this as a pretext to arrest people they want to arrest anyways, well, it's a dumb pretext. They have dozens of pretexts that aren't overtly unlawful; the idea that they're going to haul people in for an offense that they won't be able to articulate when they process you at the station (because it is not in fact a violation of any California statute) seems, to me, far-fetched.
It is problematic that they ask this question. I'm not lobbying for its inclusion. It's bad because people will, as has been pointed out all over the thread (including by my top comment) believe they have to answer. But the response to that problem should be to educate people not to answer the question, not to accept the victimizing framework of the article.
The police, by the way, cannot hold you for 48 hours while looking for something to charge you with. To arrest you, they need probable cause of a specific crime, which they'll need to provide when they book you. They can then charge you with other stuff they dream up while they hold you, but they need something to start with. Again: refusing to answer this question isn't "Obstruction of justice" (which isn't a California offense in the first place); the California "resisting" statutes generally require force or violence on your part, or your interference with their questioning of someone else.