Ask your agent. "Can I Tweet our earth-shattering union because it is THE BEST THING EVER??" or something more professional.
I'm not sure why your agent might want to keep it a secret either, but it can't hurt to ask. Fwiw, with my previous agents, there was no announcement and they didn't even bother following me on Twitter, and they're very active on there. :-/ With my current one, he Tweeted it about 20 minutes after we talked on the phone, before I even signed the agreement. Well, he talked while I went, "HI YES I WANT TO WERK WITH YOU GOSH SO MUCH EXCITE." He was someone I'd worked with before though, so I knew what the agency agreement was like.