I'm a pretty reserved person, but I can't say I've ever felt lonely at such a social event. To me, the whole mood of heavily populated social events tends to be lively and happy, so I'm OK with not necessarily being in the thick of the "life-of-the-party" type stuff. If find it pleasant, whether I'm with just a couple of/few people or alone at the event.
Come to think of it, I've never felt lonely in any busy, crowded, public place. To me, it's just kind of "part of the deal" to be one individual who feels kind of separate from "everyone else" in a crowd of strangers.
Also (and not that anyone asked), I've never really felt particularly lonely when I'm alone either. lol
The only place I've noticed feeling lonely has been when I'm with someone who should know me better than they do, but don't; and as a result, talks to me as if I'm someone that I'm not. That happens to me a lot, so maybe that's why I'm content and happy either among a bunch of strangers, a crowd of people at a big social event (whether I know them or not), or alone. I most often really enjoy being with a person or two that I'm close to, but in some cases I do find I have to then have some time alone to replenish my "mental energy" and get back to feeling like I'm on solid, non-lonely, ground.
Why the thing with a person or two happens a lot is, ironically, because I'm so good with relating to people I know how to find the things we have in common. As a result, people often tend to assume I'm "just like them" when I'm really only like them in limited ways but emphasize those ways as a way of being able to relate. OR, if I show signs of not being "just like them" it can throw them off, as well as leave them "not getting me" and saying something "that doesn't apply" as a result. So, the "relating thing" is both a good social skill and my own "interpersonal downfall" sometimes. In any case, it's often less stressful for me to either be alone or else be an anonymous member of a crowd.