My experience living and travelling in Europe was that this is definitely true for TripAdvisor, but much less so for Yelp. Mostly tourist traps (that you'd find anyway) on TripAdvisor, while I found some amazing, hidden, non-touristy (think no printed menus, just staff that explains the day's dishes in the local language) restaurants on Yelp.

I concluded that Yelp is used more by locals, but maybe the people using it just align with my tastes better.