What should we do about network-effect monopolies?

Even on the margin, anything that costs Facebook users also makes it less valuable for its remaining users—it’s a negative feedback loop. The same goes for any other site where users create value for other users, like Twitter or Craigslist or Yelp or Wikipedia. (It’s not an accident that these are some of the most stagnant popular websites!)

If I look at Wikipedia I don't buy that Wikipedia's stagnation is due to lack of money. The problem is more that the Wikimedia Foundation is that constant war with the community and doesn't spend it's money wisely. 

The WMF would also have no problem with being a membership organization that charges members money. It would however mean that it would become democratically accountable which the WMF abhors. 

How would a standardized social network work? Maybe it would let you download your friends lists and posts in a format that could be easily uploaded to a different social network? That way there's less switching costs for social networks.

I think there are different sets of "we" that you might be addressing the question to. The instinct to use government is understandable, but without an understanding of what is actually monopolized, it's unlikely to be effective. Note also that the network effect is a recognition of a whole lot of value being created, at least some of which is NOT captured by the monopoly. Simply reducing the effect (by limitation or forced disaggregation) could easily hurt consumers a lot, in addition to hurting the monopolist.

For some values, forced interoperability (published least-common-denominator APIs and specifications) can help, but they're slow and the providers are very well trained in how to extend them to preserve their value-add in protected(monopolistic) ways.

And the fundamental network-effect value is _very_ hard to open up. That value is identity/addressing. The exact right balance of (feeling of) privacy and accessibility is fragile and not very portable.

The examples that you highlighted in your links are Google Chrome, Facebook, Grubhub, Amazon, Apple phones, Google search, and Yelp. Out of these, FB is the only one that seems like it deserves to be called a monopoly: Chrome competes with Firefox (among others), Grubhub competes with Caviar (among others), Amazon competes with google searching "buy [product]" and the gazillion other results that show up, Apple phones compete with Android, Google search competes with Bing and DuckDuckGo, and Yelp competes with Google reviews as well as other review sites.

Signal started out with wanting to build a federalized and interoperable platform but after a while concluded that it's a bad idea because it siffles the ability to innovate. Email still isn't end-to-end encrypted by default. I do use Signal to communicate with my friends over using email. I likely wouldn't if their would be no desktop version of Signal (the breaking change that ended their interoperability). 

On the more hopeful side, Protocol Labs does good work with IPFS, OrbitDB, AvionDB and FileCoin. Microsoft providing an open protocol with ION is also good news. 

On the standardization and interoperability side of things. There's been effort to develop decentralized social media platforms and protocols. Most notably being the various platforms of the Fediverse. Together with opensource software, this let's people build large networks that keep the value of network effects while removing monopoly power. I really like the idea of these platforms, but due to the network monopoly of existing social media platforms I think they'll have great difficulty gaining traction.

Even on the margin, anything that costs Facebook users also makes it less valuable for its remaining users—it’s a negative feedback loop.

I think you meant to say "positive feedback loop". "Negative" refers to self-stabilizing, not bad/undesirable or the sign of the change.

Is that average ad revenue a mean or a real average?