The figures from internet research firm Hitwise show that Bing’s market share fell from 9.48% to 8.96% last month, despite the $80m advertising budget allocated when the search engine was launched by Microsoft in June.
Yahoo! also saw a fall in market share from 16.96% to 16.38% in September while Google’s share rose from 70.24% to 71.08% over the same period.
Microsoft and Yahoo! are awaiting regulatory approval for a proposed 10-year search deal that would see Bing’s search engine technology power the Yahoo! Search, with Yahoo! selling ad space.
If the deal goes ahead it would pose a greater challenge to Google than either brand has achieved separately.
Last month Yahoo! revealed a $100m marketing campaign to promote its new open platform web-hub and brand strategy.
Earlier this week Google announced the launch of a new feature that would allow advertisers to serve richer ads to take advantage of the growing use of smartphones. Google also announced it has appointed Torsten Schuppe to the new role of marketing director, UK and Ireland.