I never understood why so many people love BBM? especially It can only be used on Blackberry devices. In my opinion better services exist that don't tie you in to owning a phone from one manufacturer.
Have a look at WhatsApp, it's basically a better version of BBM for any mobile platform worth owning: http://www.whatsapp.com/
The advantage of WhatsApp is you install the app, it integrates with your phone book meaning if you have someones number and that person has WhatsApp installed you can both message on that without needing to add each other as friends. If WhatsApp was to go bust tomorrow you would still have all your contacts in your phone book to SMS / Call.
If you got an Android phone for Email you have the Android Email app which will do push email from Hotmail / Outlook & Exchange (maybe others too) and you have the Gmail app for all things Gmail. You get the official Facebook app, along with Facebook messenger which will give you push notifications of Facebook messages allowing you to chat just like on BBM. For Twitter you get the official Twitter app for Android which i think it fine for my basic Twitter use, it will also give you notifications about events if you wish.
Android 4.0+ devices such as the Galaxy Nexus can also be encrypted and ask for a pin before the device will even boot, so you can secure your info on the device.
As for an Android device that's like a Blackberry i can only think of the HTC Cha Cha
which to be honest i don't think was ever that great. I would get one of the Google Nexus phone, you will get used to a touch screen keyboard in no time.