User activity search is available on profile pages.
posted by pb to Feature Requests at 1:50 PM (78 comments total)
In the upper-right corner of profile pages, MeFi members will see a new option to search that user's activity. Throw in a search term, click the button, and you'll find matching posts and comments with that term from that particular user. Happy searching!
As with full site search
and favorites search, this doesn't handle exact phrases very well. So if you're searching for an exact sentence rather than keywords, a Google search with the added phrase "posted by [username]" might still be your best bet.