I think it's one of those situations where virtually any rule you could implement would disadvantage someone.
I'd contrast it with Trump's election fundraising total of $255m, vs Hillary's $512m. (as of October 2016)
How important is it in winning an election? .. Where Hillary had the support of Hollywood, Wall Street and most of the media, Trump had huge grassroots support, who'd rally and post on message boards.
Corbyn is really the Trump in this election (the chaotic populist – in Zizek's words – who divides his own party and who most don't take seriously as a politician).
The amount of online support Corbyn gets from unpaid devotees would be very difficult to quantify against May's campaigning .. From many online polls, you'd think the Tories weren't going to win a single seat .. The fact is many Conservative voters (myself included) don't broadcast our views outside the voting booths, because they've become so demonised in the public space .. I'd say this is certainly far from fair – but again, maybe impossible to regulate.