While it does sound a bit manipulative to link you a card through her site where she knew you'd likely see her posts on depression etc., maybe she is afraid to ask anything of you? Sounds like both of you are a bit afraid to communicate on some things. Which I'm sure is hard, trying to figure out the way you both want the open adoption to go.
I think sending a card to remember her for Mother's Day is a good idea. Doesn't mean it would take away anything from you on your day with your son. You could mail it on Friday or something like that. (maybe next year I mean) It is your day to be celebrated with your son, and no reason to feel guilty for getting that day. That's not your issue to deal with...her pain, I mean but taking charge of her pain and acknowledging it or being empathetic are two different things. Doesn't need to overshadow your day or place.
Just my observance from years of being on here, but the 1st year is really difficult for both of you, just different reasons. Give yourself time to work things out and adjust to how your relationship will grow.