I need to build a social network that is expecting very high traffic, 10k - 50k + registrations in a day, not even including visitors, pageviews, image uploads, etc..., a lot of database activity. We have a solid quad core dedicated server right now with 8GB ddr3 and 2tb hdd sata. Will doctrine be to slow as far as queries and load time to handle such a high traffic website?
Short Answer: You will certainly worsen performance, and hide database specific features that could provide more performance.
Suggestion: Use MongoDB - and a Database only where highly relational - transactional ACID compliant data is required. No ORM needed. Orm's hide the implementations of the underlying data-store. This is a great thing and an awful thing at the same time. We use MongoDB and Postgres at my corporation; This will not change for a long time. Our code is easy to read, to the point, and doesn't rely on "middleware". When new Postgres 9 features became available we could take advantage immediately. When the Python MongoDB driver was updated we could refactor very easily - no need to hack the ORM layer to accomplish this.
If you are building something like this you will need a combination of databases. A text search database, a relational database, and a fast schema-less database perhaps? How does an ORM help solve this problem? - it doesn't.