It appears that a "VaryByCustom" parameter is missing from the cache configuration for that page. MonoX server side caching (do not mix it with client-side browser cache) can operate on both page and Web part levels. "Let's socialize" page by default has page-level (output) caching turned on to minimize the impact of loading user avatars, blog posts and groups. You can specify different "VaryBy" parameters to control how it behaves - it basically wraps the default ASP.NET functionality described at this URL: http://msdn.microsoft.com/en-us/library/hdxfb6cy(v=VS.100).aspx
By default, MonoX varies the output cache by the application ID, language, user roles (since administrators and other groups usually have a totally different view of a same page), and a target page URL. In this case, since the page holds a user name, it must be varied by user also.
You have several choices here: to turn the page cache off, to add caching on the Web part level (in the Web part properties) or to add the varybycustom user parameter. Page cache is controlled in the page management section (/MonoX/Admin/PageAdmin.aspx, you must be logged in as an admin), find the page in the right pane (its title is Social Networking), right click on it and choose Page properties. You will be able to see the Cache tab in the page properties screen, so either set Cache duration to 0, or enter user in the Vary by custom field.