In rare move, Facebook founder attended meeting to assure Omnicom, Dentsu Aegis Network and Unilever as it faces growing advertiser boycott.
It is first in package of tools that will be rolled out in coming months.
Q1 ad revenue growth was just 0.4%.
Ad revenue growth now appears to have stabilised.
Internal email reveals internet advertising giant is also implementing hiring freeze.
Users will be able to discover and drop in to rooms, as well as use AR filters and custom backgrounds.
Top news, insights and analysis every weekday
Sign up for Campaign Bulletins