What is best software for big membership site?

Hey everybody I want to make a good membership site that doesn't have anything to do with IM or anything like that.

It will have the look and feel of a dating site where people can upload their pics and have profiles and such. And where they can search users based on location and other things whichever kind of users they are looking for on my site. So it would essentially be setup like a dating site. I want to know what software or script I can get that will do everything I need. But I don't want to pay for those stupid build a dating site stupid monthly paid programs. I want to be able to install it on my hosting provider and administrate it myself.

Plus I would like to know about hosting. If my site happens to go viral and I end up with users in the millions how can I be safe with my hosting company? How do huge sites host there site ?

Thanks,