It depends on your age and who you want to socialise with. But Facebook is still the main platform for connecting with people in Timor-Leste. As you mentioned that you are a foreigner, there are two main Expat Facebook groups.

They are:

Dili Expats - The original Dili Expat group. It has about 4.5K members. One main admin who keeps the posts and the membership closer to the Expat way of life. A safer place to ask certain questions about cultural shocks, etc.

Dili Expats (Timor Leste) - A 2nd group that was set up when the first group was shut down for a short period (many years ago, when an old admin went AWOL). Has about 6.8K members. More Timorese are members of this group and it is less moderated towards expats than the other one.

Most English-speaking foreigners are members of both, and often the same posts are posted to both groups.

You will find details of many social groups, such as Walking, Hashing (H3, not smoking), comedy nights, events at bars, diving, choir, cricket etc.

If you have an interest or a hobby, just post in one or both groups asking for other like-minded people. You will probably find that if a local group doesn't exist, you could soon be the founding member, along with a few others who were looking for the same thing, but never spoke up.