I think it is better to build your own brand name rather than relying on another one. An 'autoupdating FAQ system' is a better description/tagline than calling it Posterous for customer support. Posterous is famous but what if a small online flower retailer wants to use your product and he has no idea what Posterous is? It is better to directly explain what your product does rather than relating it to another one.
On the other hand, if a tech blog writes PilarHQ is the posterous for customer support, that is well and good!