Blocking a rival social network from building up its social graph by using your own is a tactic that echoes the most feared atomic weapon of the 1980s - and which Twitter is deploying ruthlessly

In the 1980s, the neutron bomb was the new thing to be Worried About. Plain old atomic weapons which devastated cities and their populations were passé; now the idea was that warring sides would fight it out by detonating weapons which released a devastating neutron blast, which would rapidly kill the population (from radiation poisoning) yet leave the buildings standing, and (in comparison to conventional atomic weapons) not very radioactive at all. Because of its intended design to kill people while leaving the valuable property standing, the neutron bomb was also known as the "capitalist bomb".

Well, it's taken a little while, but Twitter is now unleashing its own neutron bomb on other social networks around it.

The latest to feel its effects is Tumblr, which has been blocked from letting Twitter users search for their friends on the blogging network. Tumblr thus joins LinkedIn and Instagram in being banned from piggybacking on Twitter's social graph, and expectations are high that Flipboard (whose chief executive left Twitter's board this month, not happily) is next.

Tumblr, for its part, said it is "truly disappointed by Twitter's decision… to our dismay, Twitter has restricted our users' ability to 'Find Twitter Friends' on Tumblr", it said in a statement to Techcrunch. ""Given our history of embracing their platform, this is especially upsetting. Our syndication feature is responsible for hundreds of millions of tweets, and we eagerly enabled Twitter Cards across 70 million blogs and 30 billion posts as one of Twitter's first partners. While we're delighted by the response to our integrations with Facebook and Gmail, we are truly disappointed by Twitter's decision."

Why is Twitter doing this? If you think, it's quite obvious. It doesn't want other social networks, whether longstanding or fast-rising, to piggyback on its social graph - the web of interconnections of who follows who which, if you mine it well, tells you about peoples' interests (aha! You follow Will.I.Am and Nicki Minaj, so there's just a faint chance you're interested in R&B) and so helps you build not just an engaging social network, but also to get a leg up on successfully monetising it, especially through adverts.

And so, it deploys the Twitter neutron bomb. Boomph. No more authorisation of requests from those fledgling (or larger) social networks. It leaves the infrastructure standing - all those database tables waiting to be filled up with peoples' names and connections - but prevents the creation of connections between them. The "social" part of the network is prevented from growing.

Twitter's in a special place in the social network field, too. It's smaller than Facebook, and perhaps even Google+, but its users tend to be more active and to range more widely. That makes them, and their social graph, much more valuable to a wannabe social network (hello, Tumblr) that wants to connect people easily; if you can essentially transplant your Twitter friends from Twitter to Tumblr, then there's a good chance you'll spend more time on Tumblr. That, of course, is not what Twitter wants, because less time spent on Twitter means less attention paid to Twitter ads.

What Twitter is saying to Tumblr, Instagram, LinkedIn and any number of other social networks that want to build themselves into attractive places to go is: good luck with that. We did it the hard way; now it's your turn. Those empty buildings of your social network? Yeah, pity, that. I'll bet there won't be more than one social network able to build its "connections" database from Twitter's by the end of the year. The exception? Posterous - Twitter bought it in March this year. (Pinterest is still in there; if Twitter allows it to stay in the "approved list" then either it is eyeing it up for acquisition, or it thinks it isn't a threat.)

This strategy should be obvious enough that it's surprising that anyone is surprised by it. Twitter, with a billion dollars in venture capital backing already sunk into it, isn't actually in the business of giving away its social graph. And we've seen an earlier version of this battle play out before. Remember how in 2008 Google cut off the then fast-rising social network Facebook's access to its Google Contacts API, so that you couldn't import email addresses from Gmail into your (new) Facebook profile? Google said that Facebook would have to provide a reciprocal API back out - something which Facebook had banned when it stopped the use of Google's Friend Connect, intended to let people export their Facebook contacts.

The intriguing point about that, of course, is that in retrospect Google was trying to piggyback on Facebook's social graph, even as Facebook was trying to do on Gmail's. Google at that point wanted them for Google Buzz. Yet even after Buzz ran aground, the row with Facebook went on - and in retrospect, it's obvious that Mark Zuckerberg didn't trust Google not to be trying to build its own social network and using Facebook's social graph to do it. Which turned out to be wise: imagine if Google had been able to mine the Facebook social graph to create Google+. The neutron bomb tactic looks the same everywhere; all that's required to deploy it is to have become the Silicon Valley equivalent of a nuclear nation - and these days, that means a social network with user figures in the multiple tens of millions.

Twitter's latest deployment of the social network neutron bomb isn't the first, and likely won't be the last, but it's probable that it's going to mark a turning point. If wannabe social networks can't piggyback on Facebook, or the non-API-offering Google+, and not now Twitter either, it may suddenly become extremely difficult to get the scale you need to break through. We may have seen the last of the big new social networks. A capitalist bomb indeed.