One of the earliest iPhone apps to make a name for itself in restaurant recommendations with its trademark fruit machine slots-type interface, Urbanspoon also offers a service called Rezbook, which allows restaurant owners to manage bookings from Urbanspoon users via an iPad app.

Today, the Seattle-based startup has announced that 1000 restaurants have signed up to the service, while online reservations processed by Urbanspoon have increased by 325% since January this year.

Rezbook pitches itself against OpenTable, the online booking service with over 12,000 restaurants around the world on its books. Although it’s around only a twelfth of the size of OpenTable, UrbanSpoon’s service launched in May 2010, while its larger rival debuted twelve years earlier in 1998. As a result, it would seem that Rezbook is growing nicely.

It’s worth noting that Urbanspoon hasn’t relesased any figures for restaurants using the Rez widget it offers, allowing users to book from their websites, so the gap between the two companies may actually be smaller. However, the requirement for an iPad may put some smaller restaurants off signing up to Rezbook, and convincing restaurants to switch allegiances from its larger rival could be a challenge for UrbanSpoon as it looks to grow further.

In all, Urbanspoon’s restaurant recommendations app, available on multiple platforms, now boasts 20 million downloads and over 25 million monthly visits. The company says its users have made one billion ‘shakes’, the app’s method for displaying new dining suggestions.