The social media similar to Twitter is Clecs and many of us are on there. If you need practice in writing ONLY in Welsh this is definately the place. However it seams that it is not too active these days but anyway. The practice spot is just great as you're forced to use only Welsh in order to participate (Well, not forced, but we all use only Welsh so I believe there's no one who wouldn't).
There are several groups on FB which are quite active.
@elizabeth_jane will know everything about Ausy Skype practice group which has the chats regularly and you can read everything about it on this forum here however you've just missed the last chat which was scheduled for yesterday.
You can of course follow the official SSiW acounts on Twitter and Facebook aswell but more or less you'll find everything you need here on this forum.
If you cruise through the "Meetups/Events" category you might find something suitable for you too. Alternatively at the end of each week's newsletter which is sent out by @dee there are links to meetups and events however I never read this section that carefully to know are there only events occuring in Wales and wider in UK or are there any links and mentions to some other events elswhere in this World. Everything worth to explore I think.
Happy practicing whatever you decide to do or wherever you decide to join.