I use Zurker all the time (written a hub about- thanks for the reminder I need to update it!) I find it a great place to hook up with interesting people from all over the world. It now has (from memory) about 375,000 open accounts worldwide.
The main attractions for me:
Spam tightly controlled
Owned by its members
Simple but effective appearance and functionality
Over 18's only and it has a grown-up feel to it
You control your data
Great approach to privacy and freedom of speech
Tiny compared to FB- difficult to get your family and friends to join
No search or chat facility (under development)
On balance I'd say definitely worth a look- just Google "Zurker Invite" to find an invite.