Apologies for the break in service yesterday. It is still continuing as I write, but the fix is in and problems should disappear as soon at it gets copied into the production set-up, which cannot happen before the start of normal business hours in the UK.
The source of the problem is a doozy.
Yesterday this stopped working. We are not sure why, but suspect it had to do with our rapid growth in usage as yesterday we had over 25,000 different visitors to the site. We don't seem to be breaking their terms of (free) service, but maybe their automated systems decided we were some kind of attack.
Anyway, we have removed AddThis from our page, including instead simple static links to the social networks. The reason you cannot see this just yet is precisely because we did anticipate heavy demand: we have more than one server, accessed through a specialized "load balancer", and users won't see the fix until it is copied to the server clone.
We are an academic project aiming to provide a popular service to a wide audience that is free of charge to both map libraries and map users, and funded by a relatively small one-off project grant. That means we need to keep our own system as simple as possible and rely as much as possible for infrastructure on "free" services like Google Maps, Google Sites, Blogger, UserVoice ... and AddThis. However, it does make us dependent on others.