~ Talking about Texas hold 'em, there's really nothing to memorize there. If you mean you can memorize with what cards a particular player was raising/calling in that rare case when he/she decides to show cards when finally folding or losing, those cards are most likely they want you to memorize, to mislead you about their way of playing.
If Texas hold'em in Facebook, that is one old game that I played before with my friends. I can't memorize the cards, but many of my friends can do that. Perhaps, that can work for some people and not for others because playing poker games needs many things to win, and that is not easy.I've never played poker on Facebook, but I'm pretty sure if it's Texas hold 'em then the rules are the same. I still see no point in memorizing cards, sorry. If you could share just one example where memorizing was helpful, that would be great.
If that is poker online, we can not use the way we use offline poker, and we need to find another way to trick the opponent.
Bet sizing can be much more important than your face expression. An additional, although weaker factor, is time taken before your action. Both, online a offline poker, are equally suitable for these "tricks".
Maybe not memorizing but calculating odds can help.
depending on the cards on the table and on your hand you can try and calculate the odds of other players to have better hands than you.
definitely not easy, but feasible.
nice trick on checking response times of users.