The BBC has published the Conservative Party’s recent invoices from Facebook. The headline is that as of the Autumn, the party was spending at least £100,000 a month on Facebook advertising.

The key news is really in the detail, though, not the total. Ahead of spending on specific by-election campaigns, gathering support in the 40/40 target seats or attracting Likes for the main Conservatives page was “Email Collection”.

Online campaigning is about a combination of two things – access and trust. Simply broadcasting your message makes very little difference unless you can be sure you’re delivering to persuadable voters, and that they may be willing to read and digest what you say. Plus the two can be difficult to achieve together – barging into a private space uninvited may give you access to your audience but it can also instantly sacrifice any chance of trust.

Both criteria mean there’s a classification of different platforms.

Twitter is easily mistaken for a good place to win votes – it’s a constantly roiling mass of opinion and debate where feedback or viral uptake of your tweet is instant. But many a campaign has made the mistake of taking that at face value. In reality, Twitter users tend to follow people they already agree with – and a huge amount of the noise is generated by politicians, commentators and journalists, which is different to proof that floating voters are listening. It’s therefore a good gladiatorial forum to display your skills and sometimes cut through to the mainstream media, but it’s not somewhere where a lot of potential voters are making their polling day decision. The Scottish Yes campaign, and before them Yes2AV, were taken in by the positive affirmation they got on the Twittersphere, without realising that it was all coming from an echo chamber of their own existing supporters. They wasted time and resources on talking to people who already agreed with them, and they both lost.

Facebook is better. Your timeline isn’t made up of strangers who you broadly already agree with, it’s made up of friends and family who you have selected precisely because you trust them. A user is likely to be far more open in that setting, for obvious reasons. The gold standard is therefore getting ordinary users to share your content to their nearest and dearest – effectively providing their personal endorsement for your message. But that’s difficult to do, and can only be achieved organically (if you’d like to receive ConHome content on Facebook, by the way, you can Like our page here). Placing your message into someone’s Facebook experience with advertising allows you to enter that more homely environment, but you’re still an interloper. This is why the second biggest item of Conservative Facebook advertising is aimed at getting people to Like the official Conservatives page – after only a brief paid-for invasion of the user’s personal space, after they click Like then they will receive your messages more naturally in their timeline.

All of which helps to explain why the top of the heap when it comes to digital campaigning is still email. Judging from the BBC’s invoice, over three quarters of Conservative spending on Facebook ad campaigns goes on collecting email addresses. Typically, the adverts will be focused on particular topics: are you concerned about fuel duty, about your children’s chance of buying a home, about the future of the economy? If so, sign up here. That way you not only get to send emails to someone directly (with more space to communicate your message than a Tweet or small Facebook advert) but you can also tailor the content of what you send them to address issues which they have told you interest or concern them. Plus, once delivered they can be read at any time – they don’t rely on catching you in the right split-second mood, as an advert on social media does.

In short, Twitter is a bar-room argument – entertaining but unlikely to change your mind. Facebook is a private drink with family and friends – a trusted environment, but one which it is difficult to invade without irritating people. Email is in the Goldilocks Zone between the two, not too hot and not too cold – if you’ve voluntarily subscribed to be emailed about things that are of particular interest to you, then you’re open to the message and you don’t distrust or resent it so much because it isn’t delivered in an invasive way.

We don’t know the full size of the Conservative Party’s email list (or, more accurately, the total of its various lists dealing with different topics), but we’re told that it is already a sizeable source of regular, small donations. If CCHQ is spending £100,000 a month on Facebook advertising, it isn’t just to shout messages at people – it’s because they’re betting it opens the door to a far better form of communication, and more votes as a result.