Perhaps in these heady post-IPO days Google are too busy with letting you search your own pc, developing browsers (or not) and figuring out how to leverage their IM application to bother too much about Orkut.
Probably not, but it does have big problems with reliability. More fundamentally, it's easy to go for days, weeks and even months without logging in (or having a need / desire to log in).
Jeremy's Why Google needs Orkut article is informative as to intent, however the Orkut implementation is very lacking. Rebecca Blood's Thirteen ways to save Orkut hit the nail on the head with her observation, "Orkut is too inflexible to succeed" and she goes on to suggest specific remedies which would help.
The thing I find with these kind of 'social networking' sites is that it's somehow very false. My experience tells me that data becomes useless if people don't update it, the most successful automation projects I've seen manage to automate the data collection process, i.e. don't rely on the user to do anything much. They probably don't know exactly what their social circle looks like, a far more telling picture would be achieved by analysing email, phone records, IM conversations etc. If a social networking app was profiling me then simply by telling it about anoasis.co.uk it would be able to discern my interests with some semantic analysis, people I rarely/sometimes/often defer to, people I rarely/sometimes/often criticise.
The scope of publically available data might, on collation, be so all revealing as to provoke a privacy backlash against the site which attempts to map it; maybe an opt-in system would work for some data, e.g. electoral, financial etc. Or maybe that kind of data has little utility and can be omitted. The point is that Google has the ability to analyse data and infer relationships, by taking the onus off the user then non-geeks (and non Brazilians) will be receptive if there is a benefit to them. A benefit like being the cross-application 'Contacts' section of a mail program, an IM app, a photo sharing solution. Or a cross-browser, cross-pc bookmark folder. Or a 'blogrolling' type provider of links to recommended resources.
At the moment however Orkut is in a pretty sorry state doing nothing well.