I don't think that memorising cards is needed in poker. We won't really see the cards of our opponents, except of the table is being streamed online, that we could see other cards but this usually happens with a 5 to 30 minute delay. The only thing that we need is a poker strategy to play by, for that we will need to be able to calculate our chances of winning. This should be not so difficult if we don't multiple table many games. But since many people like to play fast online with many table at once, it makes more sense to create a chart for our hands we play with and our hands we fold. Such an strategy makes it very easy for us to decide which hands we want to focus on.
Each people will have their way to play poker so perhaps,
memorizing cards will be one of many ways for them to win. Calculating the chances of winning is the other way to help us to win but not many of them can calculate with the right. We could miss the calculation if we don't have much experience playing poker card games.
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.
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.