I wou;d say it even more emphatically - you do not have any rights to infringe a trademark just because you bought a domain before the mark was registered. In the above example, you would be free to use the domain to continue selling flowers because you had acquired a trademark by use (rather than registration, but that's OK). Trying to use it in any other way would have to be done in a manner that didn't infringe on the other owner's mark for use as a social network. What's more, at least in the United States, if the mark became "famous" for its second use as a social network, you might be limited from using it for ANY use other than the one you were using for before the mark became famous.
Applying those concepts to the question the OP asked, if someone buys a domain and doesn't use it at all, they have NO trademarks rights, so any eventual use must not violate someone else's trademark. If another entity uses a mark in a way that qualifies it as famous, you may own the URL but can do nothing with it without violating the trademark rights of the famous mark.
That still doesn't mean that you can't own the mark, but your only two choices are to leave it dormant (even parking probably infringes the famous mark) or sell it to the owner of the famous mark.