I've lived in SF for going on 30 years now, and can tell you that this(moving to SF in your 20's) was true even before the first dotcom boom. Lots of folks moved here to live out "their wild years" before returning back to where ever they came from. I got really lucky in that I bought a house before the first boom, and the equity has made it possible to stay here, but there's no way I'd be able to do the same thing today. I really worry about where my kids will live when they get older.