I feel sure you didn’t really mean that 700+ users ALL tried to edit their signatures yesterday…
On the subject of the Advanced TinyMCE plugin – an explanation. All WordPress TinyMCE extension plugins I have encountered are aimed at adding extra buttons/functions to the creation of blog posts. This means they are used exclusively in the Administration – NOT the public facing site. However – there is a habit for them to be loaded on the public facing site as well. This is bad coding of course. And it is made worse now since the WordPress team made it possible for other plugins (like SP) to make use of the WP supplied TinyMCE because such bad loading is most likely to cause conflicts. This can usually be corrected with a very simple change.
Now to your site. Having taken a look there are some other issues which may well be relevant to all of this and which, in any case, would be best cleaned up as we can only really diagnose a specific issue when there are no other conflicts showing up.
This codex article explains a problem your site has, what this means and what to look for: http://codex.simple-press.com/codex/faq/troubleshooting/what-is-this-jquery-conflict/
In your case something – I suspect the theme but it may not be – is loading a very old (1.4.1 as opposed to the WP required 1.8.3) version of the main jQuery library. This is also being loaded from the googleapis.com CDN and is loaded not using the WP API so later, the proper version is also loaded. This can cause all sorts of unexpected and inexplicable problems and is, basically, plain bad.