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The main menu structure is where it would be important. Does your layout now have a menu with 100 links? If your hundred categories can be sorted into five or ten more general categories with other categories under those, it would make navigation easier. You can still keep all of your categories as first level under the domain in the URL, even if they are drop-downs (or however you want to do it) under an over-category. If the point is to get away from so many links in the header, even if they are not immediately visible, then the question is how important are all the categories. Can some be consolidated?

Hello, We ran a couple of boosted post ads in november and october to promote a survey we were running but that's it - nothing recently and the only ads listed in our campaigns are the two I've mentioned above.

Forums are open ended so you can easily end up with pages that are low in quality content and with post-panda google this could have a negative effect on the whole domain.  Forums are also targeted for link building schemes which, unless actively moderated, could end up with you linking to lots of illegal sites that could end up penalising you.