Many people do a postdoc first, but it's not uncommon to go straight to assistant professor right after getting your Ph.D., especially if you came out of a renowned school like MIT.
If you are a top grad at a top program in CS, there tends to be enough faculty spots. Some day far in the future when funding dries up and the field gets older, I assume there will be fewer open spots vs. recent PhDs, so more and more grads will have to do extended postdocs to prove themselves worthy of a faculty position.
senior lecturer? in uk = associate professor (with tenure) in usa
PhD Student
Postdoc
Lecturer
Reader
Professor
There are various higher positions like head of department/faculty and prestigious professorships, e.g. the Lucasian Chair in Cambridge.
Lecturer is normally the first 'tenure track' job people get after being a postdoc for a few years. It's the first of the permanent placements you can get. That said, postdoc is a catch-all and doesn't necessarily mean fresh grad. There are postdocs in my group who are nearing retirement and have stable jobs.