Learn French: Je crois que ce site de rencontres est parfait. - I believe that this dating site is perfect.

Verbs of thinking or believing, like croire (to believe) and penser (to think), only trigger the subjunctive when they express doubt. In an affirmative statement like Je crois que, you are stating what you consider to be a fact, so you use the normal indicative mood (est). If it were negative (Je ne crois pas que...), it would introduce doubt and require the subjunctive (soit).

Why does rencontres have an 's' at the end?

The word rencontre means 'encounter' or 'date'. In French compound nouns formed with de, the second noun is plural if the concept logically involves multiple things. A dating site is built for many dates, so it takes an 's'.

Why do we use de instead of des before rencontres?

When de connects a noun to the category or type of thing it is, we don't use the definite article (les). Un site de rencontres simply means 'a dating site'. If you said un site des rencontres, it would mean 'a site of the dates', referring to specific dates instead of describing the type of website.