Yeah but it basically lived on contracts from traditional media and ad companies, which they proactively courted. When those markets tanked, so did Yahoo.
I think Twitter is in a different position because Yahoo! has never build a bi-directional connection between producers and consumers.
Obviously, that's not limited to media and ads companies. Any kind of industry is investing in PR.
I believe Twitter could succeed as a PR company but will probably fail as a tech company.