I am looking for a platfrom , preferably for .Net but seems like Joomla is "the" option. Any suggestions ?

I am looking for a platfrom , preferably for .Net but seems like Joomla is "the" option. Any suggestions ?
Any "strong" and extendable site would normally be a bespoke solution - and generally PHP is alot of the times the language of choice ... Joomla is good for it's functionality but it is more of a generallised CMS system, whereas you would be looking for a custom solution.
Hope that helps you ......

We have a couple of dating sites both using LAMP (Linux/Apache/MySql/PHP)

I suggest LAMP, it's faster than .Net and you could use Drupal.

there is one called osDate