I’ve just tried two online/social concept-mapping apps.

• bubbl.us is seriously limited – horrible, Flash-based UI. It tries to guess at box colours, but these always revert to pus-yellow if you break links – and you have to to replace the standard links with arrowed versions. Here’s my effort.
• Gliffy is much better – decent control over a fair set of things. Here’s my effort.

I’d still much sooner draw in Adobe Illustrator and have full control over everything but the speed of creating connections, the availability of many pre-drawn shapes and themes and easy online sharing of drawings makes Gliffy a clear winner.

Update – while bubbl.us can print to PDF (at least on MacOS), Gliffy can’t. But it can export as SVG – and those are easily changed to PDFs and any other graphics format you like!