Social life
Galle is very much a small town from a social point of view. There are plenty of places to meet people, but you will tend to keep meeting the same people.
The result is that it is easy to meet people, but your friends are likely to be drawn from a very small group of people.
Places to meet people
If you have children you will of course, meet other parents very easily. There are a number of regular or semi-regular events that are worth going to.
The Sun House has fairly regular events such as cocktails and lunches.
The Amangalle has talks on variety of topics (that have ranged from preserving paintings in a tropical climate to the architecture of Galle) somewhat less than monthly.
Both The Sun House and the Amangalle have mailings lists of their events. These also mention other events of interest.
Galle is also a friendly place and, if you are even moderately extrovert, you are likely to end up talking to people at cafes, restaurants and bars. It is very useful to chat to locals: even if they do not become friends, keeping on good terms with people is a very good idea in a small community.
We welcome further suggestions.
Socialising with Sri Lankans
There are a small number of Sri Lankans from Colombo, or who have been brought up abroad, in Galle. You will tend to meet them in the same places as foreigners.
As for the real locals there is also a considerable cultural gap. One problem is that the locals who socialise in the same way as the people this site is aimed at have moved to Colombo, or out of the country altogether. It is likely to take you a while to make many friends there.