To be honest, I'm not really fussed by any of these arguments because there are far more interesting questions. The first, or the crux of the issue, is whether in reaching out to people in social media you are able to convince them, or whether you're paying for messages to reach those who would only ever vote for you, or conversely, will never vote for you.
That opens up the very interesting question that nobody is asking. This election will likely be heralded as the first election fought through social media, yet again. However, what many may well miss is that it is the first when the newspapers the Conservative party might normally expect to receive support from have gone behind paywalls -- I'm think of the News UK titles here, The Sun, The Sun on Sunday, The Times and The Sunday Times. Similarly, The Telegraph offers access to a handful of articles before a paywall is erected.
Compare this to papers supporting a central or left of centre position and you find The Guardian, The Independent and The Mirror,The Sunday People all offering unfettered access which, of course, makes the articles far more shareable on social media.
So back to the first question -- can it work? It's hard to say. because for a medium that is supposed to encourage discourse, I think most people would agree from their use of Facebook and Twitter that peoples' political views are usually pretty easily picked up and most will post accordingly. There is very little two-way discussion here. People don't really engage one another, but rather post articles and statistics which back up what they already believe. In the many years I've been chatting to contacts about politics online, I've not once seen evidence that any one of us has changed his or her mind. The truth is that the vast majority of contact through social media is broadcasting rather than listening.
Social media has been proven time and again to be a very bad predictor of political outcomes because it was originally mostly populated by young, liberal professionals. That is undoubtedly changing now, though, as anyone who has seen friends duped in to sharing posts from far right groups -- I won't dignify them with a name check here but there's one in particular you'll all probably have notice cropping up more than another. So quite where the balance is, it's hard to say. However, the evidence is that predictors using social media usually get by-elections wrong through using sentiment and buzz which predicts a leaning to the centre or left of politics that isn't always reflected in the result. So it's probably fair to say that is where social's bias tends to remain.
So, if that's where social is leaning and if the articles one can share for free are generally leaning to the centre or the left of politics, then we come to the ultimate question that arises from the Conservative's social spend.
Did they have any choice?
When the medium is arguably leaning against you and the articles that people can share for free follow a similar pattern, surely the reason that the Conservatives are spending so much in social is because they have no other choice?