A survey found that 42% of the public think there is no point in keeping up contact at this stage.
But the Alzheimer's Society said family visits stimulated feelings of happiness, comfort and security.
Even as the condition progresses, it said people with dementia can still hold an "emotional memory".
This means they continue to feel happy long after a visit or experience that they may have forgotten.
The charity is calling on people to visit friends and relatives with dementia regularly and help them take part in activities they enjoy.
In a separate survey by the charity of 300 people affected by dementia, more than half said they were no longer taking part in any, or hardly any, social activities.
And 64% said they felt isolated following their diagnosis.
Jeremy Hughes, chief executive of Alzheimer's Society, said: "After spending time with friends and family over the festive period, New Year can be a bleak and lonely time for people with dementia and their carers. It's so important for people with dementia to feel connected throughout the year.
"Spending time with loved ones and taking part in meaningful activities can have a powerful and positive impact, even if they don't remember the event itself. We're urging people to get in touch with us and find out how we can help you stay connected."
A survey of more than 4,000 members of the public indicated that 68% would still visit someone with dementia who no longer recognised them.
However the charity says that people's busy lives often mean they don't manage to follow up on these good intentions, leaving many living with dementia feeling isolated.
There are around 850,000 people with dementia in the UK.
Spending time with loved ones with dementia is important even after they fail to recognise the faces of friends and family, a dementia charity says.