We have 36,844 users on a dedicated. Those features you mention can probably be managed by plugins or custom code. re: searching expertise -> you might find this of interest -> http://community.elgg.org/mod/groups/topicposts.php?topic=324462&group_guid=52477 --- we're doing pretty much just that functionality there.

Elgg's search is quite lacking at the moment, but we've completely rewritten it for 1.7.  1.7 betas / pre-releases will start toward the middle of November, with 1.7 final due by the end of the year.

I have looked at different social engines and found that elgg is also easier to use for the end user (when compared to Joomla jomsocial and vbulliten). 

Since you are looking to install for a company, you might find the skills widget interesting: http://community.elgg.org/pg/plugins/c.west/read/151952/skills-review-widget

Another great feature of elgg is that the users can create their own groups of friends (not to be confused with user groups) and address those groups directly without others seeing the content.